diff --git a/hutool-core/src/main/java/org/dromara/hutool/core/text/CharSequenceUtil.java b/hutool-core/src/main/java/org/dromara/hutool/core/text/CharSequenceUtil.java
index 15e4ac29a..af9617c45 100644
--- a/hutool-core/src/main/java/org/dromara/hutool/core/text/CharSequenceUtil.java
+++ b/hutool-core/src/main/java/org/dromara/hutool/core/text/CharSequenceUtil.java
@@ -3305,65 +3305,60 @@ public class CharSequenceUtil extends StrValidator {
 	}
 
 	/**
-	 * 方法注释: <br>
-	 * 〈大写对应下标字母〉
+	 * 大写对应下标字母
 	 *
 	 * <pre>例如: str = name,index = 1, return nAme</pre>
 	 *
-	 * <p>搜索关键字:</p>
-	 * <tag>大写</tag>
-	 * <tag>大写下标</tag>
-	 * <tag>大写字母</tag>
-	 * <tag>大小写</tag>
-	 *
 	 * @param str   字符串
-	 * @param index 下标
-	 * @return java.lang.String 字符串
+	 * @param index 下标，支持负数，-1表示最后一个字符
+	 * @return 字符串
 	 */
-	public static String upperIndex(final CharSequence str, final int index) {
-
-		if (str == null) {
+	public static String upperAt(final CharSequence str, int index) {
+		if (null == str) {
 			return null;
 		}
 
-		if (str.length() > 0) {
-			final char charredAt = str.charAt(index);
-			if (Character.isLowerCase(charredAt)) {
-				return subPre(str, index) + Character.toUpperCase(charredAt) + subSuf(str, index + 1);
-			}
+		// 支持负数
+		final int length = str.length();
+		if (index < 0) {
+			index += length;
+		}
+		if (index < 0 || index >= length) {
+			return str.toString();
 		}
 
+		final char c = str.charAt(index);
+		if (Character.isLowerCase(c)) {
+			return subPre(str, index) + Character.toUpperCase(c) + subSuf(str, index + 1);
+		}
 		return str.toString();
-
 	}
 
 	/**
-	 * 方法注释: <br>
-	 * 〈小写对应下标字母〉
-	 *
-	 * <pre>例如: str = NAME,index = 1, return NaME</pre>
-	 *
-	 * <p>搜索关键字:</p>
-	 * <tag>小写</tag>
-	 * <tag>小写下标</tag>
-	 * <tag>小写字母</tag>
-	 * <tag>大小写</tag>
+	 * 小写对应下标字母<br>
+	 * 例如: str = NAME,index = 1, return NaME
 	 *
 	 * @param str   字符串
-	 * @param index 下标
-	 * @return java.lang.String 字符串
+	 * @param index 下标，支持负数，-1表示最后一个字符
+	 * @return 字符串
 	 */
-	public static String lowerIndex(final CharSequence str, final int index) {
-
+	public static String lowerAt(final CharSequence str, int index) {
 		if (str == null) {
 			return null;
 		}
 
-		if (str.length() > 0) {
-			final char charredAt = str.charAt(index);
-			if (Character.isUpperCase(charredAt)) {
-				return subPre(str, index) + Character.toLowerCase(charredAt) + subSuf(str, index + 1);
-			}
+		// 支持负数
+		final int length = str.length();
+		if (index < 0) {
+			index += length;
+		}
+		if (index < 0 || index >= length) {
+			return str.toString();
+		}
+
+		final char c = str.charAt(index);
+		if (Character.isUpperCase(c)) {
+			return subPre(str, index) + Character.toLowerCase(c) + subSuf(str, index + 1);
 		}
 
 		return str.toString();
@@ -3378,7 +3373,7 @@ public class CharSequenceUtil extends StrValidator {
 	 * @return 字符串
 	 */
 	public static String upperFirst(final CharSequence str) {
-		return upperIndex(str, 0);
+		return upperAt(str, 0);
 	}
 
 	/**
@@ -3389,7 +3384,7 @@ public class CharSequenceUtil extends StrValidator {
 	 * @return 字符串
 	 */
 	public static String lowerFirst(final CharSequence str) {
-		return lowerIndex(str, 0);
+		return lowerAt(str, 0);
 	}
 	// endregion
 
diff --git a/hutool-core/src/test/java/org/dromara/hutool/core/text/CharSequenceUtilTest.java b/hutool-core/src/test/java/org/dromara/hutool/core/text/CharSequenceUtilTest.java
index 03b920dc8..b5efdc0df 100644
--- a/hutool-core/src/test/java/org/dromara/hutool/core/text/CharSequenceUtilTest.java
+++ b/hutool-core/src/test/java/org/dromara/hutool/core/text/CharSequenceUtilTest.java
@@ -117,6 +117,12 @@ public class CharSequenceUtilTest {
 		Assertions.assertEquals(41, v.getBytes(CharsetUtil.GBK).length);
 	}
 
+	@Test
+	void subPreTest() {
+		final String pre = CharSequenceUtil.subPre("abc", 0);
+		Assertions.assertEquals(StrUtil.EMPTY, pre);
+	}
+
 	@Test
 	public void startWithTest() {
 		// https://gitee.com/dromara/hutool/issues/I4MV7Q
@@ -337,32 +343,31 @@ public class CharSequenceUtilTest {
 	}
 
 	@Test
-	public void upperIndexTest() {
-
+	void upperAtTest() {
 		final StringBuilder sb = new StringBuilder("key");
 
-		final String s1 = CharSequenceUtil.upperIndex(sb, 0);
+		final String s1 = CharSequenceUtil.upperAt(sb, 0);
 		Assertions.assertEquals("Key", s1);
 
-		final String s2 = CharSequenceUtil.upperIndex(sb, 1);
+		final String s2 = CharSequenceUtil.upperAt(sb, 1);
 		Assertions.assertEquals("kEy", s2);
 
-		final String s3 = CharSequenceUtil.upperIndex(sb, 2);
+		final String s3 = CharSequenceUtil.upperAt(sb, 2);
 		Assertions.assertEquals("keY", s3);
 
 	}
 
 	@Test
-	public void lowerIndexTest() {
+	void lowerAtTest() {
 		final StringBuilder sb = new StringBuilder("KEY");
 
-		final String s1 = CharSequenceUtil.lowerIndex(sb, 0);
+		final String s1 = CharSequenceUtil.lowerAt(sb, 0);
 		Assertions.assertEquals("kEY", s1);
 
-		final String s2 = CharSequenceUtil.lowerIndex(sb, 1);
+		final String s2 = CharSequenceUtil.lowerAt(sb, 1);
 		Assertions.assertEquals("KeY", s2);
 
-		final String s3 = CharSequenceUtil.lowerIndex(sb, 2);
+		final String s3 = CharSequenceUtil.lowerAt(sb, 2);
 		Assertions.assertEquals("KEy", s3);
 	}
 }