Abstract: COWS (Crude Oil Washing System) technology developed By Taiho Company of Japan is used to mechanically wash oil tank with the same oil stored in the tank, without changing the quality of the oil, and at the same time, all the oil can be recovered. This technology, with a high recovery rate of oil and beneficial to safety and environment, is a best selection to wash oil tank. The paper introduces the working principle and operation procedure, evaluates the washing results of the technology used in 6002# oil tank in Huangdao tank farm.