看起来我已经整理好了。诀窍是在服务上添加Accept头：

var header:Object=new Object();

      **header["Accept"] = "application/json";**

        service.contentType = "application/json";
        service.headers = header;
        service.send('{}');

我希望它能对某人有帮助祝你好运。

谢谢，这对我很有帮助。我将标题分配简化为：
httpService.headers = { Accept:"application/json" };

我想我应该发布一个更清晰的例子。
--------------------------------------JsonHttpService.as

package services
{
    import mx.rpc.http.HTTPService;
    import mx.rpc.http.SerializationFilter;

    public class JsonHttpService extends HTTPService
    {
        private var jsonFilter:JsonSerializationFilter = new JsonSerializationFilter();

        public function JsonHttpService(rootURL:String=null, destination:String=null)
        {
            super(rootURL, destination);
            resultFormat = "json";
        }

        override public function get serializationFilter():SerializationFilter {
            return jsonFilter;
        }
    }
}

---JsonSerializationFilter.as

package services
{
    import mx.rpc.http.AbstractOperation;
    import mx.rpc.http.SerializationFilter;

    public class JsonSerializationFilter extends SerializationFilter
    {
        public function JsonSerializationFilter() {
            SerializationFilter.registerFilterForResultFormat("json", this);
            super();
        }

        override public function deserializeResult(operation:AbstractOperation, result:Object):Object {
            return JSON.parse(result as String);
        }

        override public function getRequestContentType(operation:AbstractOperation, obj:Object, contentType:String):String {
            return "application/json";
        }

        override public function serializeBody(operation:AbstractOperation, obj:Object):Object {
            return JSON.stringify(obj);
        }
    }
}