The unit testing tool Junit mainly deals with fundamental class testing and compounding class testing. However, it cannot conveniently test private methods and domains, neither cannot support testing the method, whose type of parameters is a class, in a fundamental class or a compounding class. In addition, it even cannot efficiently support some special Java class testing, such as the inherited class, abstract class and interface. This paper discusses how to introduce new testing principles and a new plug-in to Junit, and how to extend the source code of Junit to improve the testing effectiveness.