来自Apple的 NSUserDefaults文档objectForKey ：

返回的对象是不可变的，即使您最初设置的值是可变的。

From Apple's documentation for NSUserDefaults objectForKey:
The returned object is immutable, even if the value you originally set was mutable.

该行：

dictionary = [NSKeyedUnarchiver unarchiveObjectWithData:data];

丢弃先前创建的 NSMutableDictionary 并返回 NSDictionary 。

discards the previously created NSMutableDictionary and returns a NSDictionary.

将加载更改为：

NSData *data = [[NSUserDefaults standardUserDefaults]objectForKey:@"Key"];
dictionary = [NSKeyedUnarchiver unarchiveObjectWithData:data];

完整的例子，也没有必要使用 NSKeyedArchiver 在此示例中：

Complete example, there is also no need to use NSKeyedArchiver in this example:

NSDictionary *firstDictionary = @{@"Key 4":@4};
[[NSUserDefaults standardUserDefaults] setObject:firstDictionary forKey:@"Key"];

NSMutableDictionary *dictionary = [[[NSUserDefaults standardUserDefaults] objectForKey:@"Key"] mutableCopy];

dictionary[@"Key 1"] = @0;
dictionary[@"Key 2"] = @1;
dictionary[@"Key 3"] = @2;

for (NSString * key in [dictionary allKeys]) {
    NSLog(@"key: %@, value: %@", key, [dictionary objectForKey:key]);
}

NSLog输出：

键：键2，值：1

键：键1，值：0

键：键4，值：4

键：键3，值：2

NSLog output:
key: Key 2, value: 1
key: Key 1, value: 0
key: Key 4, value: 4
key: Key 3, value: 2