Abstract: This paper introduces the quality control of large saddle steel structure in the ship hull sectional preassembly welding, and puts forward the method of using three-dimensional simulation technology to guide the saddle welding precision quality control of large marine LNG tank, and implements it in the construction process of 7000PCTC car-rolling ship. The implementation results show that after the saddle welding, the precision quality is fully able to meet the late LNG tank epoxy pouring, and the LNG tank hoisting is in place at one time. It can significantly improve the production and construction efficiency of ships, shorten the ship construction cycle, and improve the construction environment. The research results of this paper can be used in the high precision welding quality control of the saddles of LNG storage tanks of similar ships.