Abstract: To accurately design the control system of scroll expander, two groups of nonlinear differential equations were established resptively between the valve opening and inlet pressure, as well as between the internal power and rotating speed, based on mechanism analysis of the expander and static mathematical relationships among the inlet pipe, inlet chamber, expansion chamber and exhaust chamber, etc. By analyzing the relations between the inlet pressure and internal power, a dynamic model was obtained between the valve opening and rotating speed. Simulation results show that the dynamic behavior of the model is in good accordance with that of the scroll expander, which proves the model to be accurate and effective.