Abstract: An analysis model on the two-stage compression refrigeration/heat pump system with vapor injection system was set. The optimal volume ratio of high-pressure cylinder to low-pressure had been discussed under both cooling and heating conditions. On this basis, prototype was set up and experimental studies had been done. The results show that,compared with single-stage compression system,the cooling capacity and cooling EER(energy efficiency ratio) can increase 5%- 15% and 10%- 12%,respectively; the heating capacity with evaporating temperature ranging from 0.5 to 3℃ is 92%- 95% of that under the rated condition,and 58% of that under the rated condition when evaporation temperature is between- 27.6 and- 26.1℃.