The working cycle theory model is the theoretical and important basis for the study of the suction and compression performance of all kinds of vacuum pumps and compressors. Due to large deviation in the actual performance of the liquid ring compressor, it is difficult for the traditional theory based on the "suction-compression-discharge" ideal working cycle to guide practical application. Therefore, an actual working cycle model was established for liquid ring compressors in this paper, which considers the effect of the residual gas after discharge. The rationality of the theoretical model was verified by the external characteristic performance test of a series of liquid ring compressors (Type 2BE1) commonly used in industry. This model provides a theoretical basis and method for analysis of suction compression performance of liquid ring compressors and has a certain scientific research and application value.