//检查用户名是否注册
				var isRegist = "";
				$.ajax({
	                type: "POST",url: "registUsernameCheck.do",dataType : 'text',data : $('#form1').serialize(),async: false,success: function (data) {
	                    if (data == 1) {
	                    	$("#usernametip").html("");
	                    	isRegist = "";
	                    }
	                    else {
	                    	$("#usernametip").html("<img src='<%=request.getContextPath()%>/images/error.png'/><font color='red'>该用户名已经被注册！</font>");
	                    	isRegist = "registed";
	                    }
	                }
	            });
				
				if(isRegist.length == 0){
				}else{
					return false;
				}

把异步处理设置 为false；即可给方法外部赋值：

<span style="color:#cc0000;">  async: false,</span>