Abstract: To overcome the problems of control programs which were programmed according to conventional modes, such as the poor replantability and difficult maintenance, a new approach is put forward to use FSM to develop CNC system softwares. The basic operation principle of FSM is described and a modeling scheme involving hierarchical FSM is proposed for intricate systems. Discusses the main software modules used in open CNC system software and their FSM behavior models. Then, the system restructuring mechanism based on FSM is introduced, which makes FSM available to define the functions of software components and control the partitions of logic so as to improve the flexibility in system restructuring. Furthermore, the way how to extend the function served to readjust adaptively the feed speed is studied on the basis of system restructuring mechanism of FSM.