Software has been used more and more widely in CNC systems, the defects caused by software among all defects of a CNC device have been higher and higher as well. How to diagnose the defects caused by software in CNC device fast and effectively is the topic of this paper. This paper talks about the features of embedded software first, and then introduces a proved method used in software test and defect diagnosis by using software system test platform. We’ll use an instance to elaborate on it. We’ll make a general summary of this method at the end of the paper.