There are many shortcomings in the open CNC system control program developed by the traditional programming mode, such as maintenance difficulties and poor portability. The application and development of FSM in CNC system are researched in this paper, and the basic principles of FSM and reconstruction mechanism of FSM are introduced. The reconstruction process based on FSM by the application of hierarchical modeling method and status table are also constructed. At last, the adaptive control function of automatic adjusting feeding speed in three axis CNC milling machine is extended to realize the function definition of the software unit in control system and control logic separation.