问jQuery .css()不适用于后台url

EN

            function requestbg(cusa) {
                $.ajax({
                    url: 'api/bg.php',
                    type: 'GET',
                    data: {id: cusa},
                    success: function(response) {
                        if(response != "") {
                            var url = response;
                            $("body").css("background-url", url);
                            console.log(response);
                        }
                    }
                });
            }