Abstract: The hydraulic safety valve is a indispensable safe device for the vertical tank, although some defects of this kind of valve were found in its use, provided that the newly-modified breather valve is substituted for the hydraulic safety valve, the more tank failures would take place. The paper analyzes in detail the escape of this valve's sealing fluid during the breathing in and out, and points out that the main cause of the escape is induced by the valve's structure. Therefor, a new-type hydraulic safety valve has been designed, its oil-gas separator is a hemicycle tube in which the flow direction of gas with sealling fluid droplet is changed in innumerable times when the gas is flowing within it, which makes the oil and gas separate in innumerable times, so the sealing fluid can not escape and loss any more.