The invention provides an air-conditioning system and a control method thereof. The air-conditioning system comprises a refrigerant circulating loop and a controller, wherein the refrigerant circulating loop is mainly formed by connecting a compressor, a reversing valve, an indoor heat exchanger, an outdoor heat exchanger and an electronic expansion valve through a refrigerant pipeline; and the controller is used for controlling the air-conditioning system to operate. By virtue of the air-conditioning system provided by the invention, a refrigerant heater is additionally arranged between the outdoor heat exchanger and the electronic expansion valve of a throttling device; the refrigerant heater is controlled to be started, and the frequency of the compressor and the openness of the electronic expansion valve are correspondingly controlled according to actual working conditions, so that under the condition of non-reversing of the reversing valve in a heating mode, a frost layer on the outdoor heat exchanger can be molten; and meanwhile, the indoor heat exchanger continuously supplies heat to a room, the comfort of the air-conditioned room is ensured, and the air-conditioner using experience of a user under the low-temperature working condition is improved. The air-conditioning system is simple in structure, easy to control, reliable to operate and easy to implement. Compared with like products, the air-conditioning system has higher competitive advantages.