Abstract: In order to solve the problem of the unequal between internal and external pressure ratio in single screw refrigeration compressor under different environmental conditions and improve the efficiency, compound slide valve capacity control mechanism was proposed in this paper. The compound slide valve capacity control mechanism can achieve capacity and internal volume ratio regulation simultaneously. On the basis of the operating feature of single screw compressor, the structural parameters of a single screw refrigeration compressor were determined, and then the influences of the starting position of the by-pass and string gas groove on internal volume ratio under different load conditions were analyzed. A model was established to calculate the effective by-pass area, which laid the foundation for the study of the part-load working characteristics, and the influences of different part-load conditions and slide valve starting position on effective by-pass area under different load conditions were obtained. Results show that the compound slide valve can achieve internal volume ratio regulation under full-load condition, and internal volume ratio increases with the decrease of capacity under part-load conditions. String gas groove can ease its increase effectively. Jumping value increases with the closer of slide valve starting position to exhaust end. Effective by-pass area and duration increase with the decrease of load. In order to increase the performance of part-load conditions, the starting position of by-pass should be as close as possible to inlet end, but the starting position of by-pass can't cross the closed helix.