The LNG stratification and rollover may occur due to the density difference in the process of storing and filling in large LNG storage tank.The phenomenon directly results a large amount of boil off gas (BOG) and sharp pressure increase inside the tank. The safety valve releases a host of BOG, thereby, the explosive LNG heavy gas cloud is formed around the LNG storage tank.When it encountered ignition source, a heavy explosion will happen.Based on FLUENT software, a two-dimensional mathematical physics model was established to simulate the stratification and rollover mass and heat transfer process of the LNG storage tank.The main factors of stratification and rollover are found out by studying the initial density difference, the critical density difference and rollover coefficient.The result shows that the main cause of rollover is the LNG initial density difference.The larger the initial density difference, the easier it is to rollover.The reasonable way to fill the LNG storage tank is to select same or similar component and quality, enhance tank insulation to reduce heat leakage.The critical density difference and rollover coefficient can be seen as effective criterion.The results of this paper have important guiding significance for preventing tanks from rollover and early warning with effective measures.