The recent research development on the hot-gas defrost for air-source heat pumps was reviewed. An ideal defrost model was presented based on the fundamental conservation equations and experimental results. The distributed parameter method was served to study the details of the heat and mass transfer at the frost side and the pressure variation at the refrigerant side during the reverse cycle defrost. Good coincidence between simulation results and experimental data shows that the dynamic defrost model might be effective in the research of the heat pump especially in the study on dynamic characteristic of reverse cycle defrost.