Abstract: While a computer control system is designed, it is very difficult to use the stability criterion of discrete control system-the distribu-tion of the roots in z plane-to judge the stability of the system if there aredifferent sampling times in the system. Therefore, it is a effective method to use simulation to judge its stability. This paper mainly discussed a practical variable speed control system with rotation and current closed-loops. By analyzing the system model and choosing the method of simulation, it gave out the result of the system simulation in the end.