Abstract: In order to investigate the internal leakage characteristics of the expander, a mathematical model of the thermodynamic working process and a two-phase leakage model for gas-oil flow were established. The influence of clearance height on the leakage path of different leakage path was analyzed, and the influence of rotational speed and inlet pressure on the volumetric efficiency and leakage flow rate under three organic fluids (i.e. R123, R134 and R245fa) was compared. The results show that the leakage through the clearance gap between screw rotor and shell accounts for the most part, followed by the leakage through the clearance gap between screw groove and gate rotor, and the leakage through the clearance gap between gate rotor and shell is relatively small. The volumetric efficiency increases with the rotational speed and the inlet pressure. The volumetric efficiency of the single screw expander using the R123 as the working fluid is the best under the same working conditions, followed by R245fa and R134a.