Abstract: The seasonal variations in condensing ambient conditions have a significant impact on the performance of the organic Rankine cycle system and the operating conditions of the expander. Using a new single screw expander with quasi two-stage expansion is an excellent way to ensure efficient cross-season operation of organic Rankine cycle under variable condensing ambient conditions. In this paper, a coupled model of quasi two-stage expansion single-screw expander and organic Rankine cycle system under variable condensing ambient conditions is developed to reveal the relationships of condensing environment conditions, single-screw expander performance and optimal operating point of organic Rankine cycle. At the same time, the inner volume ratio of quasi two-stage single screw expander is optimized. The results show that the introduction of velocity expansion process can reduce the optimal inner volume ratio of the expander and improve the optimal isentropic efficiency of the expander. The optimal inner volume ratio decreases from 6.0 to 4.86, and the corresponding optimal isentropic efficiency increases from 0.679 to 0.692. The net power and the net efficiency of ORC both have optimal values. The net power is up to 13.87kW when the wet bulb temperature is 5℃ with the inner volume ratio of 4.0, and the net efficiency is up to 12.66% when the inner volume ratio is 8.0. The maximum net power and net efficiency of ORC can be improved by 4.51% and 0.59% respectively by using the new single screw expander with quasi two-stage expansion.