Abstract: To solve the problems of air-source heat pump running at low temperature condition that heating performance is reduced and the exhaust temperature rises too fast, the circulation principle of vapor-injected air-source heat pump system was proposed by using injection subcooling and a R32 experimental system was conducted. The variation of heat pump system with sub-cooling mass flow, vapor injection pressure and evaporating temperature was discussed under low temperature heating conditions. The compression system of the R32 vapor-injected heat pump was also compared with the single-stage compression system and two-stage compression system with vapor injection, by using injection subcooling. Results show that it gets lower discharging temperature. Compared with the single-stage compression system, the discharging temperature drops by 28℃ to 32℃, the heating COP increases by 34.4% to 41.2%. Compared with the two-stage compression system with vapor injection, the discharging temperature is reduced by 10℃ to 18.24℃, whereas the heating COP is reduced by 3.5% to 8.9%.