Aimed at the problem of the compressors of the low temperature air source heat pump system, this paper analyzes the impact of volume ratio on performance and proposes a novel three-cylinder two-stage variable volume ratio rotary compressor. The performance of the proposed compression system is compared with that of the traditional two-stage compression system of the same terminal in the experiments. The results show that the three-cylinder two-stage system operates in a stable manner with a coefficient of performance (COP) of 1.52 at a ambient temperature of -30 ℃,while the traditional two-stage system does not work. The COP of the three-cylinder two-stage system is always 1.25% to 12.41% higher than that of the traditional two-stage systems at any ambient temperature. When the ambient temperature is stable and the water supply temperature increases, the amount of dissipated heat at the terminal increases. At the same time, the maximum heat of external machine decreases, as well as the COP. When the ambient temperature is 7 ℃ and -25 ℃ respectively, and the water supply temperature changes from 40 ℃ to 55 ℃,the COP of the three-cylinder two-stage system is 1.15% to 8.86% and 4.32% to 7.33% higher than that of the traditional two-stage system, respectively. The power consumption of the three-cylinder two-stage system is always 3.78% to 16.67% lower than that of the traditional two-stage system.