Abstract: This paper studied the back-pressure loops in the hydraulic system of an injection molding machine, and several models were established by an AMESim software through changing the damping hole and the damping ratio of relief valves. The back pressures of the pilot valve and main valve were analyzed under different conditions, which included spring stiffness, damping holes and its sizes. The simulation results indicated that a proper reduction in spring rate for the pilot valve could reduce the rise of pressure in the preplastic back pressure system. When the diameter of the damping orifice was reduced from 2 mm to 0.5 mm, the pressures of the upper and lower chambers in the pre-plastic back pressure valve varied within 0.02 MPa. The rise of pressure no longer took place. The damping hole not only reduced the rise of pressure in the back pressure valve but also improved the stability of the hydraulic control system.