diff --git a/rc-service/rc-service-order/order-application/src/main/java/com/red/circle/order/app/common/V5PayOrderNoUtils.java b/rc-service/rc-service-order/order-application/src/main/java/com/red/circle/order/app/common/V5PayOrderNoUtils.java
index 2dd169b4..cf64acfa 100644
--- a/rc-service/rc-service-order/order-application/src/main/java/com/red/circle/order/app/common/V5PayOrderNoUtils.java
+++ b/rc-service/rc-service-order/order-application/src/main/java/com/red/circle/order/app/common/V5PayOrderNoUtils.java
@@ -41,12 +41,12 @@ public final class V5PayOrderNoUtils {
 
   private static String normalizedPrefix(String prefix) {
     String value = Objects.toString(prefix, DEFAULT_PREFIX).trim();
-    String normalized = (value.isBlank() ? DEFAULT_PREFIX : value).toUpperCase(Locale.ROOT);
-    if (!PREFIX_PATTERN.matcher(normalized).matches()) {
-      // 无分隔订单号若允许数字前缀会产生命名空间碰撞；符号和 Unicode 同样不符合支付方格式。
+    String rawPrefix = value.isBlank() ? DEFAULT_PREFIX : value;
+    if (!PREFIX_PATTERN.matcher(rawPrefix).matches()) {
+      // 必须在大小写转换前校验原文；否则 ı/ß 等 Unicode 可能折叠成 ASCII 并与合法命名空间碰撞。
       throw new IllegalArgumentException("V5Pay order prefix must contain only ASCII letters");
     }
-    return normalized;
+    return rawPrefix.toUpperCase(Locale.ROOT);
   }
 
   private static boolean isValidLocalOrderNo(String localOrderNo) {
diff --git a/rc-service/rc-service-order/order-application/src/test/java/com/red/circle/order/app/common/V5PaySignUtilsTest.java b/rc-service/rc-service-order/order-application/src/test/java/com/red/circle/order/app/common/V5PaySignUtilsTest.java
index 49a27ec8..83e6a5f9 100644
--- a/rc-service/rc-service-order/order-application/src/test/java/com/red/circle/order/app/common/V5PaySignUtilsTest.java
+++ b/rc-service/rc-service-order/order-application/src/test/java/com/red/circle/order/app/common/V5PaySignUtilsTest.java
@@ -48,6 +48,15 @@ public class V5PaySignUtilsTest {
     assertInvalidOrderNo("1ASLAN", "23");
   }
 
+  @Test
+  public void unicodeCaseFoldingCannotAliasAsciiNamespace() {
+    Assert.assertEquals("ASLANI23", V5PayOrderNoUtils.providerOrderNo("aslani", "23"));
+    Assert.assertEquals("ASSLAN23", V5PayOrderNoUtils.providerOrderNo("asslan", "23"));
+    // Java 大写会将 dotless-i 与 sharp-s 折叠成 ASCII；原文校验必须在转换之前完成。
+    assertInvalidOrderNo("aslanı", "23");
+    assertInvalidOrderNo("aßlan", "23");
+  }
+
   @Test
   public void orderParserRejectsOtherNamespaceEmptyAndIllegalCharacters() {
     Assert.assertEquals("", V5PayOrderNoUtils.localOrderNo("ASLAN", "LALU123"));