The temperature test system for the floating roof oil tank is established. Four different kinds of test tubes are invented to test the temperature profile in the oil tank. Based on the test system, the temperature profile of a 10 × 10⁴ double-deck floating roof oil tank is investigated. There are three low temperature regions near walls in the tank, surrounded by these regions is the high temperature region in the center of the tank. The temperature distribution in different regions is presented in detailed. With the changing of ambient temperature, there is the corresponding temperature fluctuation of oil in the surface layer. With the ambient temperature increasing, the influence range decreases. And as the variation of ambient temperature increases, the temperature fluctuation of oil increases. Moreover, the heating process of oil in the tank by the entering oil is presented by the test data. Based on the principle of conservation of energy, the total heat transfer coefficient of oil tank on different working conditions is calculated with the value from 1.09 W/m² °C to 2.32 W/m² °C.