Abstract: The RTCP function of the 5-axis machine tool works normally, it is necessary to calibrate the relevant geometric parameters of the machine tool correctly. The accuracy of calibration directly affects the RTCP operation accuracy of machine tools. It is an important part of high-grade CNC system to realize the automatic calibration of RTCP parameters of machine tools through the probe, which can effectively improve the efficiency of RTCP parameter calibration of machine tools and ensure the accuracy stability of machine tools. A method based on least square fitting is proposed to realize the automatic and high-precision calibration of RTCP parameters of machine tools, which has the advantages of high calibration accuracy, fast calibration speed and no need of human interference in the whole process. This function is realized on the machine equipped with GNC60 numerical control system. Finally, the validity of the method is proved by checking the RTCP accuracy of the machine tool and the machining accuracy of the S-shaped specimen after the automatic calibration cycle.