At present,the replacement and maintenance of CRDM in in-service unit of PWR nuclear power plant in China,is mainly monopolized by AREVA,and no company has the capability to replace CRDM in in-service unit of PWR nuclear power plant.Therefore,the development of a complete progress of in-service unit replacement for CRDM is of great significance. This paper mainly introduces the automatic welding operation essentials and technical parameters of the lower sealing welds involved in the replacement of the CRDM in the PWR reactor unit in service. According to the process of stand-by test,the proper operation parameters are given,which can provide the reference for the following replacement of the CRDM in the PWR reactor unit in service.