Abstract: Ventilation capacities of the four mechanical breathing valves in oil tanks are analyzed.It is Dointed out that gravitation and spring valves utilize constant work principle, with small flow toefficient。which can ensure safety pressure of the oil tank and reduce oil loss, while two—stage precursor and mB: gnetic contro breathing valves utilize discharge work principle, with large flow coefficient, which can efficientlv guarantee the breathing safety of the oil tank.