需要这样设置 ：

    AFHTTPSessionManager *manager = [AFHTTPSessionManager manager];
    manager.requestSerializer = [AFHTTPRequestSerializer serializer];
    //    AFPropertyListResponseSerializer AFHTTPResponseSerializer AFJSONResponseSerializer
    /**
     这两个 必须设置
     */
    manager.responseSerializer = [AFPropertyListResponseSerializer serializer];
    manager.responseSerializer.acceptableContentTypes =[NSSet setWithObjects:@"application/json", @"text/json", @"text/javascript", @"text/html",@"text/plain",nil];
    
    //设置请求头 压缩
    [manager.requestSerializer setValue:@"gzip" forHTTPHeaderField:@"Accept-Encoding"];
    [manager.requestSerializer setTimeoutInterval:15];
    

另外一种方法

    AFHTTPSessionManager *manager = [AFHTTPSessionManager manager];
    manager.requestSerializer = [AFHTTPRequestSerializer serializer];
    //    AFPropertyListResponseSerializer AFHTTPResponseSerializer AFJSONResponseSerializer

    manager.responseSerializer = [AFHTTPResponseSerializer serializer];
    manager.responseSerializer.acceptableContentTypes =[NSSet setWithObjects:@"application/json", @"text/json", @"text/javascript", @"text/html",@"text/plain",nil];
    
    
    NSDictionary *paramter = @{
                               @"UserName":@"123456",
                               @"Password":@"123456a",
                               };
    
    

    [manager POST:url  parameters:paramter progress:^(NSProgress * _Nonnull uploadProgress) {
        ;
    } success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        //需要转换一下
        NSString *receiveStr = [[NSString alloc]initWithData:responseObject encoding:NSUTF8StringEncoding];
        
        NSLog(@"%@",receiveStr);

        
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
        NSLog(@"error ===: %@",error);
    }];