The invention provides a pressure boosting device of a hydraulic system. The pressure boosting device comprises a hydraulic motor, a hydraulic pump, an oil tank and an energy accumulator, wherein oil flows into the motor from a pipeline to drive the motor to rotate; the pump pumps the oil from the oil tank into the energy accumulator, a certain amount of pressure is inflated into the energy accumulator in advance, an output shaft of the motor is connected with an input shaft of the pump, the hydraulic motor rotates to drive the hydraulic pump to rotate, and the discharge capacity of the hydraulic motor is larger than that of the hydraulic pump so that the large discharge capacity motor drives the small discharge capacity pump to convert low-pressure oil from a pipeline into high-pressure oil to be stored in the energy accumulator. By means of the pressure boosting device of the hydraulic system, surplus energy such as potential energy in the system can be conveniently converted into hydraulic energy to be stored, and the hydraulic energy is supplied to the system when the system needs more energy, so that environment-friendly and economical effects are achieved; the stored energy can be used as a high-pressure source to provide high-pressure oil for the hydraulic system, so that addition of a single high-pressure pump is avoided, the size of the system is reduced, and costs are reduced.