The utility model relates to an automatic dewatering device of oil storage tank, being equipped with floater, the bottom of density between oil and water and being equipped with the toper groove with the sealed jar body, a jar internal portion including the differential gauge can laminate with the toper groove completely in the floater bottom, and toper tank bottom portion intercommunication has a drain pipe, and a jar body top communicates respectively to be had into oil pipe and return oil pipe, the drain pipe, is equipped with the valve on advancing oil pipe and returning oil pipe respectively, the drain pipe with advance oil pipe on still be equipped with the flowmeter respectively, two measuring junctions of differential gauge are located respectively inside jar internal portion and the drain pipe. The the device good reliability, the dehydration is accurate, and is with low costs.