Abstract: The paper introduces a method to reduce the evaporation loss of large outer floating-roof tank. This method uses recirculated water to form a low temperature water-layer on the surface of the floating roof, can keep the temperature of floating roof in certain range. It can avoid the rising of oil temperature due to the sun radiation and the rising of surrounding temperature in summer, and finally reduces the oil pressure and evaporation loss. The paper provides construction program and technical data on how to retrofit and design, and evaluates the method's application. This method has the advantages of simple construction, low cost, low water consumption. The method is suitable for the floating roof tanks in the central and south region in China, especially for the volatile oil tanks.