In order to fill the domestic technology gaps and improve the capability of self-maintenance of main equipment in nuclear power plants,the automatic welding technology for replacing in-service heater elements of voltage stabilizer is researched. According to the characteristics of replacing in-service electric heater and the shortcomings of existing technology,the closed welding head is developed,which improves the stability of welding arc and gas shielded effect. Through a large number of automatic welding technology tests,the influence of protective atmosphere,specimen size and the chemical composition of base metal on the appearance of weld are discussed,and the welding parameters are determined. The results show that the use of argon backside protection and helium front protection welding can ensure complete penetration and protective effect on backing pass;the deviation of specimen structure size can affect the appearance of weld due to different heat dissipation;and the higher the content of S in the base metal chemical composition,the easier it is to penetrate.