To resolve the problems aroused by G code in manufacturing industry, a plan for NC system based on STEP-NC is proposed. The whole system is designed into two parts: system software and system architecture. With modularized design thinking, the control software includes five modules: interpreting module, man-machine interface, task coordinator, axis group module and axis module. For the system architecture, an open architecture is used which adopts different products from different manufacturers, being connected together by softSERCANS bus. By a machining test, the expected machining features are realized, proving that the NC system designed is reliable.