function iframeReady(iframe, fn) {

//iframe 如果 加载完成了

    if (iframe.attachEvent) {

        iframe.attachEvent("onload", function () {

            fn && fn.call(iframe);

        });

    } else {

//iframe 如果 加载未完成

        iframe.onload = function () {

            fn && fn.call(iframe);

        };

    }

}

调用：

$("#landPrePermissionFrame").attr("src", "http://"+CONSTANT.netServiceAddrIp+"/AppDemoLigerUI/FlowRedirectProxy.aspx");

    var $myIframe = $('#landPrePermissionFrame');

    iframeReady($myIframe[0],function psg(){

        var $myIframe = $('#landPrePermissionFrame');

        var params = {

         eventType:"flowRedirect",

         param:{

             pageName:"用地预审收件"

         }

     };

     $myIframe[0].contentWindow.postMessage(JSON.stringify(params),

     "http://"+CONSTANT.netServiceAddrIp+"/AppDemoLigerUI/FlowRedirectProxy.aspx");

    });