Model based web application testing is a crucial approach in software testing. The ASM model is based on the presentation layer of web applications, and presents interactive, dynamic and low coupling features of web applications via source code analysis. The test case generation based on the ASM model, allowing for testing the unexpected behavior of users, is expanded by adding the states of invalid access and the edges of invalid transition to the primary path. However, with the expansion of web applications, the space of test cases is exploded due to the increase of the states of invalid access and the edges of invalid transition. To solve the problem, the ASM model is reduced by merging the equivalent states and equivalent transitions, which are defined for ASMbased test generation through the research on the ASM. In this way, the states of invalid access and the edges of invalid transition are consequently reduced, and thus the test case space is effectively decreased. Experiments on a realworld web application show that the test case space of the ASM model is reduced by 74.38%  without decreasing the coverage of atomic sections and the number of detected errors.