Abstract: Large LNG storage tanks usually work under micro-positive pressure and low temperature. Any leakages of heat from external environment during static or dynamic operations may lead to flash vaporization of LNG. In addition to gas loss, such flash evaporation may lead to higher operation cost, and even LNG stratification and subsequent rolling, thereby resulting in higher pressure in the tank which may severely threat the tank. With consideration to characteristics of large LNG storage tanks, a simple calculation method for daily evaporation rate is given. Impacts of light on heat leakages in storage tanks are highlighted, and calculation methods of tank surface temperatures under different conditions are proposed. This method is used to calculate the daily evaporation rate of a LNG storage tank with capacity of 16×10⁴ m³, revealing that the results meet general requirements for evaporation rates of large LNG storage tanks. Moreover, an indirect method of liquid level difference is deployed to calculate actual evaporation of the storage tank, showing results in coincidence well with that obtained by the simple method. The proposed method can provide accurate analysis method and bases for assessment of insulation performances during design, installation and production of LNG storage tanks.