As a type of positive displacement expander, single screw expander (SSEs) can be widely applied in the energy storage systems and waste heat recovery field. The irreversible losses (such as leakage, flow, heat transfer, intake and exhaust pressure loss…) have great influence on the expander performance. However, irreversible flow loss in the expander is nearly impossible to investigate experimentally and theoretically. In this paper, a three-dimensional computational fluid dynamics (CFD) study of SSE using mesh deformation approach was presented. The CFD model was validated by the experimental results. Field distribution of pressure, temperature and velocity of SSE were carried out. An energy loss factor based on entropy production principle was used to measure the irreversible flow (including leakage) loss. The energy loss caused by direct dissipation and turbulent fluctuation dissipation was compared. The energy loss of different region was investigated. Results show that energy loss of the turbulent dissipation is far more than that of direct dissipation. The energy loss factor decreases from 0.547 to 0.221 when the rotation speed changes from 2000rpm to 4000rpm. The shaft efficiency increases from 39.8% to 52.1% with the internal volume ratio from 3 to 5.