Based on the principle of doublearc method, continuous twoway circular interpolation method and the control of interpolation error are used to achieve a dynamic expansion on the process step and to get contours node data that meets the requirements of CNC machine tools’ circular interpolation. Accordingly, corresponding CNC program software can be developed. Using this kind of software, the CNC process program can be gotten but only inputting several parameters of the cycloidalpin gear. In this case, automatic programming for cycloid gear of any number of teeth is realized. According to the contours node data and the track coordination of tool center, the process program of cycloid gear can be simulated. The experimental result of cycloidalpin gear CNC grinding validates the correctness of the theory deduction and practicality of the software.