diff --git a/pkg/util/ranger/detacher.go b/pkg/util/ranger/detacher.go
index bb2b74a2ee4bd..da1aadb10eed5 100644
--- a/pkg/util/ranger/detacher.go
+++ b/pkg/util/ranger/detacher.go
@@ -779,7 +779,10 @@ func (d *rangeDetacher) detachDNFCondAndBuildRangeForIndex(condition *expression
 				hasResidual = true
 			}
 			points := rb.build(item, newTpSlice[0], d.lengths[0], d.convertToSortKey)
-			tmpNewTp := convertStringFTToBinaryCollate(newTpSlice[0])
+			tmpNewTp := newTpSlice[0]
+			if d.convertToSortKey {
+				tmpNewTp = convertStringFTToBinaryCollate(tmpNewTp)
+			}
 			// TODO: restrict the mem usage of ranges
 			ranges, rangeFallback, err := points2Ranges(d.sctx, points, tmpNewTp, d.rangeMaxSize)
 			if err != nil {
diff --git a/pkg/util/ranger/ranger.go b/pkg/util/ranger/ranger.go
index ef8d30789f8ec..c59b0982ed8c7 100644
--- a/pkg/util/ranger/ranger.go
+++ b/pkg/util/ranger/ranger.go
@@ -486,7 +486,10 @@ func (d *rangeDetacher) buildRangeOnColsByCNFCond(newTp []*types.FieldType, eqAn
 		if rb.err != nil {
 			return nil, nil, nil, errors.Trace(rb.err)
 		}
-		tmpNewTp := convertStringFTToBinaryCollate(newTp[i])
+		tmpNewTp := newTp[i]
+		if d.convertToSortKey {
+			tmpNewTp = convertStringFTToBinaryCollate(tmpNewTp)
+		}
 		if i == 0 {
 			ranges, rangeFallback, err = points2Ranges(d.sctx, point, tmpNewTp, d.rangeMaxSize)
 		} else {
diff --git a/tests/integrationtest/r/planner/core/casetest/partition/integration_partition.result b/tests/integrationtest/r/planner/core/casetest/partition/integration_partition.result
index dd34b8ef6e571..44ef24389322b 100644
--- a/tests/integrationtest/r/planner/core/casetest/partition/integration_partition.result
+++ b/tests/integrationtest/r/planner/core/casetest/partition/integration_partition.result
@@ -738,3 +738,15 @@ Projection	10.00	root		list_partition_pruning.thash.a
         └─Limit	10.00	cop[tikv]		offset:0, count:10
           └─Selection	10.00	cop[tikv]		gt(list_partition_pruning.thash.a, 10)
             └─TableFullScan	30.00	cop[tikv]	table:thash, partition:p3	keep order:true, stats:pseudo
+drop table if exists t;
+create table t(col varchar(32) COLLATE utf8mb4_general_ci DEFAULT NULL) PARTITION BY KEY (`col`) PARTITIONS 7;
+explain format = brief select * from t where col = 'linpin';
+id	estRows	task	access object	operator info
+TableReader	10.00	root		data:Selection
+└─Selection	10.00	cop[tikv]		eq(list_partition_pruning.t.col, "linpin")
+  └─TableFullScan	10000.00	cop[tikv]	table:t, partition:p4	keep order:false, stats:pseudo
+explain format = brief select * from t where col = 'LINPIN';
+id	estRows	task	access object	operator info
+TableReader	10.00	root		data:Selection
+└─Selection	10.00	cop[tikv]		eq(list_partition_pruning.t.col, "LINPIN")
+  └─TableFullScan	10000.00	cop[tikv]	table:t, partition:p4	keep order:false, stats:pseudo
diff --git a/tests/integrationtest/t/planner/core/casetest/partition/integration_partition.test b/tests/integrationtest/t/planner/core/casetest/partition/integration_partition.test
index b54787a65910a..a19c49b78cfbe 100644
--- a/tests/integrationtest/t/planner/core/casetest/partition/integration_partition.test
+++ b/tests/integrationtest/t/planner/core/casetest/partition/integration_partition.test
@@ -180,3 +180,8 @@ explain format='brief' select a from trange use index () where a > 10 order by b
 explain format='brief' select a from tlist use index () where a > 10 order by b limit 10;
 explain format='brief' select a from thash use index () where a > 10 order by b limit 10;
 
+# TestIssue51316
+drop table if exists t;
+create table t(col varchar(32) COLLATE utf8mb4_general_ci DEFAULT NULL) PARTITION BY KEY (`col`) PARTITIONS 7;
+explain format = brief select * from t where col = 'linpin';
+explain format = brief select * from t where col = 'LINPIN';