To reduce the influence of each geometric error on the machining accuracy of the machine tool and improve the machining accuracy of the machine tool,the numerical control precision internal grinder was used as the research object,based on the multi-body system theory and the homogeneous coordinate transformation principle,the spatial motion error model of the grinder was obtained,and the mapping relationship between the geometric error and the motion position was established.The determination method of machining compensation point and the modification method of NC commands were studied,and the precision machining NC commands were obtained; the CNC instructions before and after compensation were obtained through the machining simulation verification of the stepped shaft specimen,and five compensation points were selected; the spatial errors before and after compensation to the ideal position were reduced from 0.616 cm,0.607 cm,0.614 cm,0.295 cm and 0.376 cm to 0.354 cm,0.398 cm,0.376 cm,0.188 cm and 0.255 cm,respectively,which were respectively reduced by 42.5 %,34.4 %,38.6 %,36.3 % and 32.1 %.The results show that the machining accuracy can be improved by using modifying the CNC instructions.