Abstract: Summarizing the characteristics of an open numerical control system, four principal problems are put forward for building the open architecture of numerical control system, i.e., the confirmation of function elements, the system description, the interaction mode of function elements and interface protocol. It follows that it is necessary to discuss how to determine the function elements of the architecture of open numerical control system and the function composition of a controller system architecture. The system characteristics and relation among function elements are explored further in view of the hierarchical control system, control flow in system, data stream and module structure. A definition is given to the interface of servo control function using C++ language from the viewpoint of implementation with the existing insufficiencies in development process summarized.