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• Air source heat pump fresh air unit with fresh and return air mixing was developed. • The supply air temperature deviation was within ±1 °C. • The heat pump COP was 2.46 when the outside temperature was −20.9 °C. • The heat pump COP was higher than that of the traditional heat recovery device. In order to ensure the indoor air quality of the building, in view of the problem of fresh air heating in severe cold regions, this paper proposes an air source heat pump fresh air unit with fresh air and return air mixing. The mixing process of the fresh air and return air is realized in the mixing box, which increases the inlet air temperature of the condenser, creates a suitable condensation pressure, and ensures the energy-saving and stable operation of the unit. In order to explore the operation effect, heating performance and energy saving of the unit, an experimental bench was built in Harbin for laboratory heating. The experimental results showed that when the return air volume was 1400 m3/h, the fresh air unit could operate continuously and stably under the conditions of ensuring the fresh air volume of 700 m3/h and the supply air temperature of 24.0 °C. When the outdoor temperature was 4.9 °C, the heat pump COP was 4.29. Even when the outdoor temperature dropped to −20.9 °C, the heat pump COP still reached 2.46, while the COP of the fresh air system combining electric preheating, heat recovery and air source heat pump was only 1.44, and the COP of the fresh air system combining electric preheating, heat recovery and electric heat was only 1.21. In addition, the heat pump COP increased with the increase of the return air volume. As the set temperature increased, the heat pump COP increased first and then decreased. [ABSTRACT FROM AUTHOR]