The invention discloses a defrosting control method for an air-conditioning system, which comprises the following steps of: defining three defrosting running modes, namely a drying mode, a general mode and a high-humidity mode; formulating different defrosting entry conditions on the three modes sequentially according to the complexity of frosting; and when the air-conditioning system is energized and operates, executing the following defrosting control processes: a, when an air-conditioning outdoor unit electrifies and enters a heating running mode for the first time, setting the high-humidity mode as the defrosting running mode; b judging whether the air-conditioning system meets the defrosting entry conditions or not, if so, executing the subsequent process, and otherwise, repeating the current judging process; c, executing the defrosting operation; and d, after the defrosting is finished, determining what kind of the defrosting running mode to be executed next time according to the current defrosting time, and returning the process b. By the defrosting control strategy, the problems of defrosting error and incomplete defrosting in high-humidity areas can be saved, the energy is saved, and the heating effect is improved.