Abstract: Nuclear vessel head is constituted by intersection structure of sphere and a given number of tubes. The welding groove is called J groove, its welding seam is irregular space curve, and its position is inside the vessel head. The welding efficiency and quality is not good with common manual welding method for J-groove. This paper proposed a multi-pass filling strategy of J-groove automatic welding process for one type nuclear power pressure vessel head. Through the analysis of welding structure, the region division of welding process and filling strategy is determined. And the selection rule of welding process parameters is put forward to control the welding appearance. The tests indicate that suitable welding appearance can be obtained through the proposed strategy.