Abstract: For some oil tanks, mechanical cleaning is unsuitable, or a lot of oil sludge is left after mechanical cleaning. Therefore, a method which combines hydraulic robot system with high-pressure water jet cleaning system is introduced. It eliminates the deficiencies of traditional manual oil sludge bagging and transportation, including heavy labor intensity, long working period and high risk level. The proposed system, with hydraulic robot as the main body and high-pressure water jet as assistant, can jet high pressure water to stir and fluidify the oil sludge at the bottom of the tank and effectively reduce the friction between the oil sludge and tank floor. This method can increase the oil sludge cleaning efficiency of robot largely and also maximize the mechanical participation and safety during the cleaning of oil tank.