The invention discloses an air conditioning system and a defrosting control method thereof. The air conditioning system comprises a compressor, a reversing assembly, an outdoor heat exchanger, an indoor heat exchanger, a defrosting branch circuit and a heat storage assembly. In the refrigeration or defrosting process of the air conditioning system, a first valve hole is communicated with a second valve hole, and a third valve hole is communicated with a fourth valve hole; in the heating process of the air conditioning system, the first valve hole is communicated with the third valve hole, the second valve hole is communicated with the fourth valve hole, one end of the defrosting branch circuit is connected with the other end of the outdoor heat exchanger, and the other end of the defrosting branch circuit is connected with the other end of the indoor heat exchanger; the defrosting branch circuit comprises a throttling assembly and a valve body assembly, and the valve body assembly is used for controlling the on-off state of the defrosting branch circuit; and in the defrosting process of the air conditioning system, the valve body assembly starts, the defrosting branch circuit is communicated, and the heat storage assembly is opened to release heat indoors. According to the air conditioning system, the defrosting process of the outdoor heat exchanger at a low-temperature environment is accelerated, the loss of indoor heat in the defrosting process is reduced, and the comfort of a room is improved.