With the increasing emphasis of the international community on low-carbon and emission reduction, LNG fuel, as the most important transitional clean energy, has received more attention. Some shipowners have begun to retrofit their fleets with clean fuel power, but there are some difficulties in the retrofitting of fuel power systems for large ships. The LNG power retrofitting of a 15 000 TEU large container ship has been examined to analyze and study several design methods for the retrofitting of the new fuel system by focusing on the LNG fuel tank, main engine, generator and boiler, gas supply system, filling system and auxiliary system. This study can provide a design method as a reference for the ship retrofitting of the new fuel power system.