A method for controlling the balanced frosting of outdoor units in a multi-split air-conditioning system solving the problem in existing multi-split air-conditioning systems of resource waste caused by a low utilization efficiency of the heating capability of outdoor units as a result of some of the outdoor units entering a defrosting state when the heating capability is not fully utilized. According to the control method, by determining whether an outdoor unit set satisfies a frosting unbalance condition, the actual output capability of outdoor units of the outdoor unit set is selectively controlled on the basis of the frosting degrees of all the outdoor units. Thus, the difference between the frosting degrees of different outdoor units in the outdoor unit set at the same time can be reduced, such that the heating capability of all the outdoor units can be fully utilized when the outdoor unit set enters a defrosting state.