Ajax is a new concept of web application development proposed in 2005. It is the acronym of Asynchronous JavaScript and XML. Once Ajax appeared, it is rapidly applied to the fields of web development. Ajax application is different from the traditional web development model, using asynchronous interaction. The client unnecessarily waits while the server processes the data submitted. So the use of Ajax can create web user interface which is direct, highly available, richer, more dynamic and closer to a local desktop application. This article introduces the main technology and superiority of Ajax firstly, and then practices web development using ASP.NET 2.0 + Ajax. In this paper, Ajax is applied to the website pass, which enables user to have better registration experience and enhances the user's enthusiasm. The registration functions are enhanced greatly as well. The experiments show that the Ajax web application development model is superior to the traditional web application development model significantly.