A three-dimensional computational fluid dynamics (CFD) simulation is conducted to study the internal flow characteristics in the internal meshing cycloidal rotor expander. The numerical simulation model is built, and the dynamic mesh technology is utilized to simulate the flow flied in the internal meshing cycloidal rotor expander. The pressure and the velocity distributions in the expander are investigated, and the isentropic efficiency is calculated. The analysis results show that the expander can be divided into three pressure levels from the high-pressure area at the inlet to low-pressure area at the outlet. The velocity of the whole flow field is relatively smooth, and the high flow velocity occurs in the gap between the inner and outer rotors and the gap between the rotors and the shell, and there are certain vortex regions in the flow field and the phenomenon of flow separation is observed. The relationship between isentropic efficiency of the expander and the rotating speed is explored. When the inlet temperature is 313.15 K, and the inlet and outlet pressures are 0.2 MPa and 0.1 MPa respectively, and the rotational speed is 3000 r/min, the isentropic efficiency reaches 50.08%.