Most of the behavior models with respect to Web applications focus on sequencing of events, without regard for the changes of parameters or elements and the relationship between trigger conditions of events and Web pages. As a result, these models are not sufficient to effectively represent the dynamic behavior of the Web 2.0 application. Therefore, in this paper, to appropriately describe the dynamic behavior of the client side of Web applications, we define a novel Client-side Behavior Model (CBM) for Web applications and present a user behavior trace-based modeling method to automatically generate and optimize CBMs. To verify the effectiveness of our method, we conduct a series of experiments on six Web applications according to three types of user behavior traces. The experimental results show that our modeling method can construct CBMs automatically and effectively, and the CBMs built are more precise to represent the dynamic behavior of Web applications.