The 6 000 m3 Liquefied Natural Gas (LNG) bunker with C-type independent cargo tank is taken as the research object, and the finite element method is used to perform the numerical simulation of temperature field in the cargo tank area with the thermal analysis module of Patran software. The conduction and convection are mainly considered during the calculation, and the calculation model is reasonably simplified. The heat transfer coefficient of heat convection is calculated according to the thermodynamic theory. The Patran software is used for modeling and finite element calculation, and the temperature field is obtained. According to the temperature field distribution, the steel grade can be established, which provides the guidance for the selection of hull structure steel plates in the cargo tank area.