A new type of air source heat pump water heater system with scroll compressor through middle injection of economizer in the course of compression is put forward operating reliably long and efficiently in low temperature in this study. The experimental prototype is set up, System fundamentals and the operation modes are introduced, Experimental research on operation characteristics of the unit in three different operation conditions is done in the lab of enthalpy difference; Heating capacity and COP were 8.84 kW and 2.43, respectively under the condition of environmental temperature of which is -7 and -8ºC, respectively for dry bulb temperature and wet bulb temperature. The result shows that the unit operates reliably long and that heating capacity and COP of the unit are improved in low temperature.