According to the structure characteristics and functional requirement of the ultra precision grinder, the hardware structure and software form of the PMAC-based open computer numerical control (CNC) system, as well as the key techniques of software development were analyzed. The software of CNC system of the grinder was developed by using Visual C++6.0 and using the designing idea of modularization. The function modules and the implementation ways of software were discussed in depth. Experimental results prove that the open CNC has full functions, which can meet the requirement of the ultra-precision grinder.