Trilateral Flash Cycle (TFC) is particularly suitable for recovering energy from low-grade heat source. This paper presented a new mathematical model for calculating the performance of a twin screw expander integrated into a TFC working with organic components. The geometric parameters related to the rotation angle of male rotor e.g. groove volume, suction and discharge port area, leakage area etc were used in the model. The combination effects of internal leakage through five paths, oil injection, gas-oil heat transfer and refrigerant property were taken into account. The sensitivity of single parameter was also analyzed. To verify the model and the calculated p-φ indicator diagram, experimental recording of the diagram of twin screw expander was performed. The results of theoretical calculation were in good agreement with the experimental data, which indicated that the model in the present paper could be used as a powerful tool for performance prediction and product development.