function selectCooperate(obj){
    var userObject=new Object();
    userObject.userString=document.getElementById(obj).value;
    var selectUrl="${ctx}/DispatchAction.do?efFormEname=UITM25&serviceName=TMTask&methodName=selectAssistantUser&orgcode=&orgcodeFilter=${data.rootorg }&taskGuid=${data.taskparentguid}&allLeader=&random="+Math.random();

    window.open(selectUrl,'newwindow','height=300,width=1000,top=0,left=0,toolbar=no,menubar=no,scrollbars=no, resizable=no,location=no, status=no');
    var a =document.getElementById("i-0-valueString").value;
       if(a!=undefined){
  
        document.getElementById(obj).value=a;
        selectCooperate_view(obj+"_view",a);
    }

}
       

function okOrgUser(){
    var users=document.getElementById("user_select");
    var userString="";
    for(var i=0;i<users.options.length;i++){
        var currentOption=users.options[i];
        if(i!=0)
            userString+=";";
        userString+=currentOption.value+",";
        userString+=currentOption.userName+",";
        userString+=currentOption.orgcode+",";
        userString+=currentOption.orgname;
    }

    window.opener.document.getElementById("i-0-valueString").value=userString;  //i-0-valueString 父页面中的文本框
    window.close();

}

var a =document.getElementById("i-0-valueString").value;
       if(a!=undefined){
  
        document.getElementById(obj).value=a;
        selectCooperate_view(obj+"_view",a);
    }

------解决方案--------------------
不要直接从子窗口操作父窗口的节点属性，尝试改为调用父窗口的所提供的函数（理解为回调函数），父窗口自己的这个回调函数来负责修改自己的值，你的这些代码也放入该回调函数中：

var a =document.getElementById("i-0-valueString").value;
if(a!=undefined){
  document.getElementById(obj).value=a;
  selectCooperate_view(obj+"_view",a);
}