diff --git a/expression/bench_test.go b/expression/bench_test.go
index f5cd8a00df..297c1ebf5c 100644
--- a/expression/bench_test.go
+++ b/expression/bench_test.go
@@ -1796,21 +1796,6 @@ func (s *testVectorizeSuite2) TestVecEvalBool(c *C) {
 	}
 }
 
-func (s *testVectorizeSuite2) TestVecToBool(c *C) {
-	ctx := mock.NewContext()
-	buf := chunk.NewColumn(eType2FieldType(types.ETString), 2)
-	buf.ReserveString(1)
-	buf.AppendString("999999999999999999923")
-	c.Assert(toBool(ctx.GetSessionVars().StmtCtx, types.ETString, buf, []int{0, 1}, []int8{0, 0}), NotNil)
-	buf.ReserveString(1)
-	buf.AppendString("23")
-	c.Assert(toBool(ctx.GetSessionVars().StmtCtx, types.ETString, buf, []int{0, 1}, []int8{0, 0}), IsNil)
-	buf.ReserveString(2)
-	buf.AppendString("999999999999999999923")
-	buf.AppendString("23")
-	c.Assert(toBool(ctx.GetSessionVars().StmtCtx, types.ETString, buf, []int{0, 1}, []int8{0, 0}), NotNil)
-}
-
 func BenchmarkVecEvalBool(b *testing.B) {
 	ctx := mock.NewContext()
 	selected := make([]bool, 0, 1024)
diff --git a/expression/expression.go b/expression/expression.go
index 2770474310..ba77f8772d 100644
--- a/expression/expression.go
+++ b/expression/expression.go
@@ -427,12 +427,9 @@ func toBool(sc *stmtctx.StatementContext, eType types.EvalType, buf *chunk.Colum
 			if buf.IsNull(i) {
 				isZero[i] = -1
 			} else {
-				iVal, err := types.StrToInt(sc, buf.GetString(i))
+				iVal, err := types.StrToFloat(sc, buf.GetString(i))
 				if err != nil {
-					iVal, err = HandleOverflowOnSelection(sc, iVal, err)
-					if err != nil {
-						return err
-					}
+					return err
 				}
 				if iVal == 0 {
 					isZero[i] = 0
diff --git a/expression/integration_test.go b/expression/integration_test.go
index c814f9c396..44eb9d2707 100755
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -6101,6 +6101,37 @@ func (s *testIntegrationSerialSuite) TestCollateDDL(c *C) {
 	tk.MustExec("drop database t;")
 }
 
+func (s *testIntegrationSuite) TestIssue15986(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t0")
+	tk.MustExec("CREATE TABLE t0(c0 int)")
+	tk.MustExec("INSERT INTO t0 VALUES (0)")
+	tk.MustQuery("SELECT t0.c0 FROM t0 WHERE CHAR(204355900);").Check(testkit.Rows("0"))
+	tk.MustQuery("SELECT t0.c0 FROM t0 WHERE not CHAR(204355900);").Check(testkit.Rows())
+	tk.MustQuery("SELECT t0.c0 FROM t0 WHERE '.0';").Check(testkit.Rows())
+	tk.MustQuery("SELECT t0.c0 FROM t0 WHERE not '.0';").Check(testkit.Rows("0"))
+	// If the number does not exceed the range of float64 and its value is not 0, it will be converted to true.
+	tk.MustQuery("select * from t0 where '.000000000000000000000000000000000000000000000000000000" +
+		"00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000" +
+		"00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000" +
+		"0000000000000000000000000000000000000000000000000000000000000000009';").Check(testkit.Rows("0"))
+	tk.MustQuery("select * from t0 where not '.000000000000000000000000000000000000000000000000000000" +
+		"00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000" +
+		"00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000" +
+		"0000000000000000000000000000000000000000000000000000000000000000009';").Check(testkit.Rows())
+
+	// If the number is truncated beyond the range of float64, it will be converted to true when the truncated result is 0.
+	tk.MustQuery("select * from t0 where '.0000000000000000000000000000000000000000000000000000000" +
+		"000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000" +
+		"000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000" +
+		"00000000000000000000000000000000000000000000000000000000000000000000000000000000000009';").Check(testkit.Rows())
+	tk.MustQuery("select * from t0 where not '.0000000000000000000000000000000000000000000000000000000" +
+		"000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000" +
+		"000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000" +
+		"00000000000000000000000000000000000000000000000000000000000000000000000000000000000009';").Check(testkit.Rows("0"))
+}
+
 func (s *testIntegrationSuite) TestNegativeZeroForHashJoin(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test;")
diff --git a/types/datum.go b/types/datum.go
index 132ce47738..725aa61670 100644
--- a/types/datum.go
+++ b/types/datum.go
@@ -1437,7 +1437,7 @@ func (d *Datum) ToBool(sc *stmtctx.StatementContext) (int64, error) {
 	case KindFloat64:
 		isZero = RoundFloat(d.GetFloat64()) == 0
 	case KindString, KindBytes:
-		iVal, err1 := StrToInt(sc, d.GetString())
+		iVal, err1 := StrToFloat(sc, d.GetString())
 		isZero, err = iVal == 0, err1
 	case KindMysqlTime:
 		isZero = d.GetMysqlTime().IsZero()
diff --git a/types/datum_test.go b/types/datum_test.go
index 1693eaa509..13dff4726c 100644
--- a/types/datum_test.go
+++ b/types/datum_test.go
@@ -72,9 +72,9 @@ func (ts *testDatumSuite) TestToBool(c *C) {
 	testDatumToBool(c, float64(0.5), 1)
 	testDatumToBool(c, float64(0.499), 0)
 	testDatumToBool(c, "", 0)
-	testDatumToBool(c, "0.1", 0)
+	testDatumToBool(c, "0.1", 1)
 	testDatumToBool(c, []byte{}, 0)
-	testDatumToBool(c, []byte("0.1"), 0)
+	testDatumToBool(c, []byte("0.1"), 1)
 	testDatumToBool(c, NewBinaryLiteralFromUint(0, -1), 0)
 	testDatumToBool(c, Enum{Name: "a", Value: 1}, 1)
 	testDatumToBool(c, Set{Name: "a", Value: 1}, 1)