Automatic welding method of nuclear island main pipes

The invention relates to an automatic welding method of nuclear island main pipes. The diameter of a main pipe is more than or equal to 325mm, the wall thickness of the main pipe is more than or equal to 40mm. In the method, a prewelding combined groove is assembled by the relative end of the pipe parent metal, and the bottoms of a root truncated edge and a lower truncated edge are bottomed and welded to form root weld passes comprising a melt-through weld pass, a first fused weld pass, a second fused weld pass, a first support weld pass, a second support weld pass, a third support weld and a fourth weld pass; filling weld is carried out on the lower truncated edge and the upper groove to form a filling weld pass; the filling weld is carried out on the lower truncated edge and the upper groove again to form a last phase filling weld pass; and covering weld is carried out on the lower truncated edge and the upper groove by adopting a swinging weld pass and/or linear weld pass. The method of the invention can effectively ensure the welding quality and the welding thickness of the root weld passes, lays the foundation for the filling weld pass, has strong welding stability, can effectively avoid non-fusion phenomenon of the interlayers and the side walls, and has relatively better overall welding quality, short-period welding and high-efficiency welding.