Abstract: Discussed is the feasibility for applying the PID close-loop control to the master cylinder oil pressure of the hydrostatic testing machine. Also elaborated are details of the modification of the master cylinder oil pressure of the testing machine’ s oil-water balance system with the PID close-loop control，involving such aspects as the testing machine’ s work flow，electrical control principle，hardware configuration and control process program，etc. Thanks to the modification， both the control accuracy and the water pressure-following response speed of the master cylinder’ s oil pressure system are enhanced so as to get the sealing ring’ s service life 3~4 times longer than before， and thus ensure the consistent operation of the testing machine.