For reducing throttling loss in a pilot oil pipe in a hydraulic system based on common pressure rail, this paper proposes a solution that increases efficiency by reducing the relieving pressure. In order to compensate for the resulting control performance degradation, the paper takes a valve controlled swing motor system under low pressure as the research aim. Considering the uncertainty of external disturbance and flow parameters, the mathematical model of the system is established, using an extended state observer to estimate the external disturbances in real time to increase the compensation value and state feedback for the robust backstepping sliding mode controller. Finally, the simulation results confirm the control algorithm proposed is effective for keeping control performance in decreasing pressure conditions.