In this paper, a soft CNCS (Computerized Numerical Control System) works on Windows-XP operating system is presented. The comparison between soft CNC and traditional CNC system is discussed. The structure of soft CNC system is introduced as follow. The realizations of each real time module, such as real time path planning and interpolation, real time close-loop controller, real time safety detection, are introduced in detail. The real time tasks are based on a real time kernel. In the end, an application of the soft CNC system is realized on a 3-axis milling machine. The machining result shows it works well.