Aiming at solving the problems of the high compressor discharge temperature, the high discharge pressure, the insufficient cooling capacity, and the reduced system efficiency in the air-conditioning system of pure electric passenger cars in the summer high temperature environment, this paper develops a low pressure economizer using the principle of the quasi-two-stage compression cycle. The heat pump air conditioning system of the air-supplemented pure electric passenger car is set up to study its refrigeration performance. The results show that in a high environment temperature of 50°C, compared with the non-supplemented system, the discharge temperature in the low pressure supplemental gas system drops by 13.54°C, the refrigeration capacity reduces by 2.0%, the compressor power drops by 3.4%, and the refrigeration coefficient increases by 1.4%.