The experiments were carried out to test the effect of air temperature and humidity on a novel PCM based reversed-cycle defrosting for air source heat pump(ASHP), and the results showed that PCM based reversed-cycle defrosting method could keep the suction pressure of compressor higher than 035 MPa,which was father higher than the system’s low suction protection pressure. The system’s defrosting time and energy consumption increased with the relative humidity when air temperature was kept constant. Furthermore, the system’s defrosting time and energy consumption increased first and then decreased with the decrease of air temperature, and the most defrosting time and energy were consumed when the outdoor air temperature was -3 ℃. It could be concluded that the air temperature and humidity affected the defrosting process greatly and the system’s reliability was improved greatly because shutting down of ASHP unit due to low suction pressure protection during defrosting could be effectively avoided by the PCM based heat exchanger, and -3 ℃ working condition could be selected as the most adverse design condition for the PCM based heat exchanger when the relative humidity was constant because the most defrosting time and energy were needed.