Pressurizer is one of the main equipment in the primary loop of PWR nuclear power plant. As the key element of pressurizer,the electric heater is welded by automatic TIG welding. In this paper,a case of incomplete penetration of the joint of electric heating element according to the original mature automatic TIG welding process is introduced. The analysis shows that that too low content of S element in the base metal changes the flow and heat transfer direction of the molten pool,and causes more welding heat to flow to the two sides of the weld,as a result,the depth of penetration becomes shallow and incomplete penetration of joint can not be solved. According to the characteristics of the existing low S base metal,the new welding parameters are adjusted through the process experiment.The two-step welding method can not only guarantee complete penetration,but also ensure that the surface depression of the final joint does not exceed the standard. Furthermore,the improvement suggestions are put forward for the welding of the electric heating element of the follow-up project and the replacement process of the electric heaters of the pressurizer in service.