The invention relates to a defrosting method of an air conditioner. The defrosting method includes the following steps that firstly, environment temperature, environment humidity and fin temperature are acquired; secondly, dew-point temperature corresponding to the environment temperature and the environment humidity is calculated, and whether fins meet the defrosting condition or not is judged according to the relation between the fin temperature and the dew-point temperature, if yes, the third step is executed, if not, the first step is executed again; thirdly, suction pressure is acquired, evaporation temperature corresponding to the suction pressure and the change rate of the evaporation temperature are calculated, if the change rate is larger than a preset change rate, defrosting is started, and if the change rate is not larger than the preset change rate, the third step continues to be executed. The defrosting method has the advantages that the change rate of the evaporation temperature of a heat exchanger on the air side in the heating process is used as the condition for judging whether defrosting is conducted or not, false defrosting of a unit is avoided, defrosting efficiency is improved, and heating capacity of the air cooling heat pump unit is exerted.