diff --git a/src/Contract/ParserInterface.php b/src/Contract/ParserInterface.php
index 83a950e..b75f31d 100644
--- a/src/Contract/ParserInterface.php
+++ b/src/Contract/ParserInterface.php
@@ -197,6 +197,15 @@ public function getArgDefine($nameOrIndex): array;
      */
     public function getArg($nameOrIndex, $default = null);
 
+    /**
+     * Get an argument value by name, will throw exception on not input
+     *
+     * @param string|int $nameOrIndex
+     *
+     * @return mixed
+     */
+    public function getMustArg($nameOrIndex, string $errMsg = '');
+
     /**
      * Set trusted argument value, will not format and validate value.
      *
diff --git a/src/Flags.php b/src/Flags.php
index 86dedfe..1a7dfb4 100644
--- a/src/Flags.php
+++ b/src/Flags.php
@@ -9,6 +9,7 @@
 
 namespace Toolkit\PFlag;
 
+use InvalidArgumentException;
 use RuntimeException;
 use Toolkit\PFlag\Contract\ParserInterface;
 use Toolkit\PFlag\Exception\FlagException;
@@ -589,11 +590,7 @@ public function hasArg($nameOrIndex): bool
      */
     public function setArg($nameOrIndex, $value): void
     {
-        $arg = $this->getArgument($nameOrIndex);
-        if (!$arg) { // not exist
-            throw new FlagException("flag argument '$nameOrIndex' is undefined");
-        }
-
+        $arg = $this->mustGetArgument($nameOrIndex);
         $arg->setValue($value);
     }
 
@@ -603,14 +600,31 @@ public function setArg($nameOrIndex, $value): void
      */
     public function setTrustedArg(string $name, $value): void
     {
-        $arg = $this->getArgument($name);
-        if (!$arg) { // not exist
-            throw new FlagException("flag argument '$name' is undefined");
-        }
+        $arg = $this->mustGetArgument($name);
 
         $arg->setTrustedValue($value);
     }
 
+    /**
+     * @param string|int $nameOrIndex
+     *
+     * @return mixed
+     */
+    public function getMustArg($nameOrIndex, string $errMsg = '')
+    {
+        $arg = $this->mustGetArgument($nameOrIndex);
+        if ($arg->hasValue()) {
+            return $arg->getValue();
+        }
+
+        if (!$errMsg) {
+            $errName = $arg->getNameMark();
+            $errMsg  = "The argument '$errName' is required";
+        }
+
+        throw new InvalidArgumentException($errMsg);
+    }
+
     /**
      * @param string|int $nameOrIndex
      * @param null|mixed $default
@@ -619,10 +633,7 @@ public function setTrustedArg(string $name, $value): void
      */
     public function getArg($nameOrIndex, $default = null)
     {
-        $arg = $this->getArgument($nameOrIndex);
-        if (!$arg) { // not exist
-            throw new FlagException("flag argument '$nameOrIndex' is undefined");
-        }
+        $arg = $this->mustGetArgument($nameOrIndex);
 
         if ($arg->hasValue()) {
             return $arg->getValue();
@@ -631,6 +642,21 @@ public function getArg($nameOrIndex, $default = null)
         return $default ?? $arg->getTypeDefault();
     }
 
+    /**
+     * @param string|int $nameOrIndex
+     *
+     * @return Argument
+     */
+    protected function mustGetArgument($nameOrIndex): Argument
+    {
+        $arg = $this->getArgument($nameOrIndex);
+        if (!$arg) { // not exist
+            throw new FlagException("flag argument '$nameOrIndex' is undefined");
+        }
+
+        return $arg;
+    }
+
     /**
      * @param string|int $nameOrIndex
      *
diff --git a/src/SFlags.php b/src/SFlags.php
index a819f58..642644c 100644
--- a/src/SFlags.php
+++ b/src/SFlags.php
@@ -96,7 +96,7 @@ class SFlags extends FlagsParser
     private $name2index = [];
 
     /**
-     * Parsed argument values
+     * Parsed input argument values
      * - key is a self-increasing index
      *
      * ```php
@@ -895,12 +895,9 @@ public function getArgument($nameOrIndex, $default = null)
      */
     public function setArg($nameOrIndex, $value): void
     {
-        $index = $this->getArgIndex($nameOrIndex);
-        if ($index < 0) {
-            throw new FlagException("flag argument '$nameOrIndex' is undefined");
-        }
-
+        $index  = $this->mustGetArgIndex($nameOrIndex);
         $define = $this->argDefines[$index];
+
         // set value
         $this->args[$index] = FlagType::fmtBasicTypeValue($define['type'], $value);
     }
@@ -911,10 +908,7 @@ public function setArg($nameOrIndex, $value): void
      */
     public function setTrustedArg(string $name, $value): void
     {
-        $index = $this->getArgIndex($name);
-        if ($index < 0) {
-            throw new FlagException("flag argument '$name' is undefined");
-        }
+        $index = $this->mustGetArgIndex($name);
 
         $this->args[$index] = $value;
     }
@@ -927,10 +921,7 @@ public function setTrustedArg(string $name, $value): void
      */
     public function getArg($nameOrIndex, $default = null)
     {
-        $index = $this->getArgIndex($nameOrIndex);
-        if ($index < 0) {
-            throw new FlagException("flag argument '$nameOrIndex' is undefined");
-        }
+        $index = $this->mustGetArgIndex($nameOrIndex);
 
         if (isset($this->args[$index])) {
             return $this->args[$index];
@@ -941,6 +932,42 @@ public function getArg($nameOrIndex, $default = null)
         return $default ?? FlagType::getDefault($define['type']);
     }
 
+    /**
+     * @param string|int $nameOrIndex
+     *
+     * @return mixed
+     */
+    public function getMustArg($nameOrIndex, string $errMsg = '')
+    {
+        $index = $this->mustGetArgIndex($nameOrIndex);
+        if (isset($this->args[$index])) {
+            return $this->args[$index];
+        }
+
+        if (!$errMsg) {
+            $define  = $this->argDefines[$index];
+            $errName = $define['name'] ? "#$index({$define['name']})" : "#$index";
+            $errMsg  = "The argument '$errName' is required";
+        }
+
+        throw new InvalidArgumentException($errMsg);
+    }
+
+    /**
+     * @param string|int $nameOrIndex
+     *
+     * @return int
+     */
+    protected function mustGetArgIndex($nameOrIndex): int
+    {
+        $index = $this->getArgIndex($nameOrIndex);
+        if ($index < 0) {
+            throw new FlagException("flag argument '$nameOrIndex' is undefined");
+        }
+
+        return $index;
+    }
+
     /**
      * @param string|int $nameOrIndex
      *
diff --git a/test/FlagsParserTest.php b/test/FlagsParserTest.php
index a83198f..3923785 100644
--- a/test/FlagsParserTest.php
+++ b/test/FlagsParserTest.php
@@ -2,6 +2,7 @@
 
 namespace Toolkit\PFlagTest;
 
+use InvalidArgumentException;
 use Toolkit\PFlag\Exception\FlagException;
 use Toolkit\PFlag\FlagsParser;
 use function get_class;
@@ -68,7 +69,16 @@ private function doTestGetOptAndGetArg(FlagsParser $fs): void
         $this->assertTrue($ok);
         $this->assertSame(335, $fs->getOpt('int-opt'));
         $this->assertSame(233, $fs->getArg('int-arg'));
+        $this->assertSame(233, $fs->getMustArg('int-arg'));
         $fs->resetResults();
+
+        // getMustArg
+        $e = $this->runAndGetException(function (FlagsParser $fs) {
+            $fs->getMustArg('str-arg');
+        }, $fs);
+
+        $this->assertSame(InvalidArgumentException::class, get_class($e));
+        $this->assertSame("The argument '#1(str-arg)' is required", $e->getMessage());
     }
 
     public function testParse_specialArg(): void