The work aims to explore the effect law of inlet pressure change on the efficiency and actual recovery power of scroll expander to reveal the effect mechanism of inlet pressure change on internal flow characteristics and performance parameters of scroll expander. With the scroll expander in the organic Rankine cycle of waste heat recovery as the research object, STAR CCM+software and overlapping grid technology were adopted to conduct the unsteady discrete solution on the internal computational fluid domain of the scroll expander, and the flow field change characteristics and the change law of performance parameters of the scroll expander were analyzed. When the outlet pressure was constant, the gradient of pressure distribution increased with the increase of inlet pressure. The flow at the inlet and outlet fluctuated periodically, and the fluctuation range of the flow at the outlet was obviously smaller than that at the inlet. The filling coefficient reflected the degree of leakage or flow resistance inside the expander to some extent. It was observed that the filling coefficient was the largest when the inlet pressure was 0.75 MPa, at which time the flow resistance of the working medium was relatively smaller and the average mass flow was also the largest. The isentropic efficiency and output power of the scroll expander increased with the increase of inlet pressure. The optimization of the output power of the scroll expander is related to a variety of factors. It is necessary to comprehensively consider the effect of the actual enthalpy difference between the inlet and outlet and the average mass flow rate, and it is not reasonable to consider only unilateral factors.