Abstract: The bypass air ratio of effective dehumidification of a heat pump drying kiln was obtained. Furthermore, the relationship between bypass air ratio and evaporation temperature as well as dew point temperature difference was investigated, and a numerical interaction model between air circulation system and heat pump system was established. Results showed that the bypass air ratio changed as a negative reciprocal function to evaporation temperature and dew point temperature difference. Numerical analysis can aid to accurately control the running status of heat pump drying unit and thus to improve the efficiency of heat pump drying kiln.