The invention discloses a cooling method of an LNG fuel tank, which comprises the following steps: 1, liquid nitrogen is filled in a liquid nitrogen tank car and transported by the liquid nitrogen tank car, wherein the temperature of the liquid nitrogen is -196 DEG C, and the liquid nitrogen supply rate is 1.2 m < 3 >/h; 2, forcible evaporation of the liquid nitrogen in the liquid nitrogen tank car is carried out by an evaporator to change the liquid nitrogen into cold nitrogen; 3, the cold nitrogen enters a fuel bin through a liquid phase pipe in a first gas station, and the cold nitrogen isintroduced into the bottom of the LNG fuel tank; 4, the cold nitrogen introduces into the bottom of the LNG fuel tank, absorbs the heat of the LNG fuel tank, and is discharged from a second gas station through a gas phase pipe after absorbing the heat; and 5, a worker continuously observes the cooling rate of the LNG fuel tank, controls the cooling rate to be 10 DEG C/h until the bottom temperature of the LNG fuel tank reaches -130 DEG C, and stops the supply of the cold nitrogen. The method is simple and convenient to operate, the design cost is effectively reduced, and the method is more suitable for the LNG fuel tank.