The utility model provides a buoyancy adjusting device for a submersible. The buoyancy adjusting device comprises a liquid bag part, a control part and an oil pipe part, the liquid bag part is connected with the control part through the oil pipe part; the liquid bag part comprises a liquid bag (1) and a liquid tank (9); a first one-way valve (27) is arranged between the liquid bag (1) and one endof the liquid tank (9); liquid is pumped away between the liquid bag (1) and the liquid tank (9) through a first one-way valve (27), and then the submersible floats upwards and dives downwards. According to the buoyancy adjusting device of the submersible, stable oil discharging under the ultrahigh pressure environment is achieved, and the oil quantity control precision is high and is 1 ml. The water surface quick oil return function of the low-pressure quick oil return system of the submersible buoyancy adjusting device is achieved, and hydraulic oil in the oil bag can be quickly pressed backto the oil tank. Compared with a traditional buoyancy system, the submersible buoyancy adjusting system saves more electricity and can reduce energy consumption of underwater equipment.