In order to expand the working range of heat pump and improve the applicable range and annual utilization rate of heat pump system, an air source heat pump unit with vapor injection technology was designed. The heat recovery device was used to recover the waste heat from the dryer exhaust gas to preheat the fresh air. The heat pump unit consisted of three sets of heat pumps with different powers. The heat pump unit’s start-stop combination adapted to different process requirements and environmental conditions, heat pump can improve the annual energy efficiency ratio by this way, and the enhanced vapor injection technology improved the performance of the heat pump system in low temperature environment. The designed heat pump unit was tested. When the ambient temperature was -5℃, the air supply temperature of the heat pump was 40℃ and the COP of the heat pump was 2.21, the air supply temperature of the heat pump used enhanced vapor injection was 45℃ and the COP of the heat pump used enhanced vapor injection was 2.33. The experimental results showed that the validity of vapor injection technology and the designed heat pump can meet the operating requirements of recirculation drier under different environmental conditions. This study was of great practical significance for the development of green and energy-saving drying process equipment system. The results showed that the heat pump dryer can be applied to rice and wheat drying operations in the Jianghuai region.