Abstract: The precise prefabrication of primary pipes is a prerequisite for the successful implementation of primary pipes automatic welding technology. Based on the geometric characteristics of the primary equipments（including RPV, SG and RCP） and primary pipes, it presents a pertinent completion size measurement scheme for calculating the processing data of primary pipes weld grooves, in order to digest the primary equipments manufacture error; and then through the development of the weld grooves processing program to ensure the machining accuracy of the weld grooves, and ultimately to ensure the precise prefabrication of primary pipes automatic welding in CPR1000 nuclear island installation.