The water pollution of oil running in the hydraulic system is a bad jeopardize to its quality that is the all-important cause of making the oil oxidation. As the hydraulic system works, the capacity of oil box will change at any moment. When the capacity decreases, the air will enter the moisture absorption after filtered. The desiccant in the absorption sorbs the moisture, then the desiccated air bursts through the entry single way valve and comes into the oil box, at the same time the atmosphere outside the oil box will close the deflated single way valve. When the capacity increases, the atmosphere inside the oil box closes the entry single way valve simultaneously opening the deflated single way valve where atmosphere is directly expelled.