Liquefied natural gas (LNG) is widely used as an efficient and clean energy source. It is common to use type B tanks to store and transport large quantities of LNG. Taking a 13000 m3 volume type B liquid storage tank as an example, under static boundary conditions, based on the conservation of mass and energy, a steady-state thermal analysis of the filling state of the type B liquid storage tank was carried out. A steady-state evaporation rate and saturated vapor pressure model was established, and a simplified calculation method using the equivalent model was proposed. The variation trend of Lng evaporation rate during the process from the initial pressure in the tank to the working pressure after LNG filling is studied. When the filling rate of the tank decreases, the higher the pressure in the tank, the lower the evaporation rate. This study has guiding value for the design and analysis of type B LNG storage tank.