All software CNC system is an effective way to realize open CNC system. Based on the analysis, the multithreading model about CNC system software was given. In the software model, the realtime control of the CNC system was finished by WDM driver of the Windows operating system, and the weak realtime with the management part about control system was implemented by the application program layer of the Windows operating system. The realtime control of CNC system in the Windows operating system was solved well. A effective method was given to build the software model. Experiment shows that the method is effective.