The invention discloses a defrosting method for a constant-frequency air conditioner. The defrosting method comprises the steps that in the heating operation process of the air conditioner, the temperature of an indoor coil and the number of times of operation of a defrosting mode in a set period are obtained; the temperature of the indoor coil is compared with a first set indoor coil temperature, and the number of times of operation of the defrosting mode is compared with a set number of times; and when the temperature of the indoor coil is lower than the first set indoor coil temperature, and the number of times of operation of the defrosting mode in the set period is larger than or equal to the set number of times, the air conditioner is controlled to operate an enforced defrosting mode for defrosting. Through application of the defrosting method, the problem that in the prior art, the heat exchange performance of an outdoor heat exchanger is weakened due to the fact that defrosting can not be conducted in time can be solved.