A low cost, anti-pollution pump/motor displacement-varying mechanism was studied in this paper. In the hydrostatic energy storage transmission system(HESTS), a variable pump (motor) unit was used to transfer the energy. A new pump/motor displacement-varying mechanism was obtained based on low price manual manipulating variable pump with high-speed on-off solenoid valves supporting pressure oil. Simulation results showed that the position precision and the ability of following sinusoidal wave signal of the new mechanism met the requirement of the pump/motor unit. Experiment results proved that the new pump/motor displacement-varying mechanism operated precisely and reliably.