Abstract: To research the internal work process of the single screw compressor based on vapor injection process, the working principle of an air-source heat pump based on single screw compressor with economizer was analyzed in this paper. The R22 was adopted as working fluid and the thermodynamic model that could describe the injection compression process was established. Taking the screw angle as the independent variable and the pressure, temperature, and the quality in the working chamber as the dependent variable, a group of one-order linearly different differential equations were set up. The fourth order Ronge-Kutta method was adopted to calculate the equations and some relevant parameters changing with the volume of the working chamber were obtained. Combining the geometric structure of the single screw compressor, a calculation model that could describe the vapor injection process and the cycle power consumption was set up, and the model could effectively calculate the relevant parameters of the single screw compressor such as heating capacity, power consumption and energy efficiency ratio at different evaporating temperatures during the vapor injection process. Through the comparison of heating capacity and the energy efficiency ratio between traditional heat pump system that has no vapor injection process and the heat pump system with economizer, it is shown that the vapor injection process has a deep influence on the working fluid in the compress chamber and the thermodynamic performance of the air-source heat pump with economize based on single screw compressor can be improved effectively.