解决方案

把ajax改为异步，用deferred

代码

function renderChildData(d) {
        var params = '&' + d.hasNewChild + '&baseTime=' + getparams().baseTime + '&period=' + getparams().period;
        var childTreeData = null;

        var deferred = $.Deferred();
        var request = $.ajax({
            url: api.childTree + params,type: 'GET',data: {},beforeSend: function () {
                $('#childLoading').html(common.loadingImg);
            },dataType: 'json'
        });
        request.done(function (json) {
            $('#childLoading').empty();
            if (json.success) {
                var data = json.data;

                childTreeData = mapTreeData(data);
                deferred.resolve(childTreeData);
            } else {
                nuit.msg({type: 'warn',msg: json.msg,closable: true,timeoutclose: true});
                deferred.reject(childTreeData);
            }
            return childTreeData;
        });

        request.fail(function () {
            $('#childLoading').empty();
            deferred.reject(childTreeData);
        });

        return deferred;
    }

     renderChildData(d).then(function (child) {
        //  这里的child 就是renderChildData 返回的childTreeData
     }