Software testing,also known as an important part of Software Development Lifecycle,will continue to be a primary approach for ensuring correctness of software systems.It is necessary to develop a software testing tool to enhance testing efficiency and reduce testing cost.In this paper,on the basis of the software testing tool for the Visual Basic language we have developed,we firstly describe the implementation of the Engine-the kernal of this testing tool,then present the architecture of the VB language analyzer which is the front end of the Engine.Concerning the VB language analyzer,we stress on the design and implementation of lexical analyzer and parser that support the software testing.Finally,we show some results that proved the completeness of the testing semantics of our testing tool from a caes study.