Abstract: We propose a rule-based expert system for computer aided control systems design(CACSD).Themain goals of this expert system are to reduce as much as possible the users' knowledge needed for operating CACSD package,and help users to obtain satisfactory design results.In this paper,we mainly expound the necessity and the importance of developing the third generation of CACSD package and present its architecture as well as the structure of the expert system and the functions of each module in thispackage.The concepts,thoughts and new ideas described in this paper may be of certain value in the development of this Package.