According to the structural characteristics of piston single valve expander in the waste heat recovery system of Rankine cycle engine, the working process simulation model was established. The test data of expander cylinder pressure under four working conditions shows that the simulated indication power error is between 0.99% and 10% and the water mass flow error is within 3% which verifies the validity of the model. Besides, the influences of inlet and exhaust structural parameters on expander performance are analyzed at inlet pressure of 5 MPa, inlet temperature of 520 °C and speed of 1 950 r/min and the changing laws of indicated power, cyclic mass flow and adiabatic efficiency of expander with the height of intake ejector pin and exhaust port were acquired.