According to the installation technologies and characteristics of the primary pipes, this technology adopts laser measurement and 3D modeling fitting technology, numerical control groove machining technology, remote control narrow gap automatic welding technology, etc., to make the primary pipe on the side of the pressure vessel welded before the steam generator is in place, thus shortening the construction period of the critical path, and solving the such problems as narrow space in the welding site of the primary pipe and rigorous requirements for welding deformation control. The developed narrow gap single-sided and double-sided groove welding processes for the primary pipe expands the welding operability and adaptability of groove machining and assembly. The unique double-pass welding method of hot welding layers avoids the defects such as welding penetration and incomplete fusion of the side wall of the hot welding layers. After the ends of the three primary pipes forming different angles of the three-dimensional space is connected with the pressure vessel, the other ends, at the same time, are accurately and freely assembled with the three nozzles of the steam generator and the main pump weighing several hundred tons; It ensures that the steam generator meets the verticality requirement after welding. This technology has been granted 15 patents (including 8 invention patents), 1 Chinese patent award, 3 provincial and ministerial level science and technology awards, and 2 provincial and ministerial normalized construction methods.