The invention relates to an outer oil bag type underwater glider buoyancy regulating device which is formed by a hydraulic cylinder cabin section, an oil circuit block cabin section and an outer oil bag. The hydraulic cylinder cabin section comprises an oil tank barrel, a piston, an inner oil tank and an oil tank front end cover. Through the control of a hydraulic system, the piston in the oil tank barrel is driven to move, and the displacement volume of the outer oil bag is controlled, so that the buoyancy regulating is realized; and the outer oil bag is in a special shape, so that the resistance on a vehicle when moving in water can be reduced.