The heat loss and surface temperature of the top surface and sidewall of large-scale floating roof tank is tested by the heat flow meter and surface temperature method. Based on the test data, the heat loss from the top surface is about twice more than that of the sidewall which means the top surface is the weakest insulation part of the floating roof tank. On the surface, the heat loss profile is in accord with the surface temperature distribution. Special attention is given on the calculation of thermal conductivity for the top surface and sidewall which finally deduced the total heat transfer coefficient of large floating roof tank. Moreover, the heat loss of floating roof tank in different working conditions is predicted. According to the calculation results, the level of 6m is regarded as a critical level determining the heat loss. And the centralization storage of oil is a more energy conservation storage pattern.