The invention provides an air-conditioning system and a control method thereof, which are used in a computer room, wherein the air-conditioning system comprises a compressor, an evaporator, a first throttling device, a refrigeration condenser, a refrigeration valve body, a first valve body, a heat tube condenser, a heat tube valve body, a first liquid accumulator, a pump body, a first check valve, a multi-passage heat exchanger, a second throttling device, a second valve body, a second check valve and a temperature controller. The temperature controller controls the opening or closing of the first valve body, the second valve body, the refrigeration valve body and the heat tube valve body according to outdoor environment temperature in order to change the flow route of heat exchange medium in the product, so that the operating mode of the product is changed, consequently, the product can be controlled to be switched between different operating modes according to the change of outdoor temperature, and thereby can utilize cold sources in the outdoor environment to the max, as a result, under the premise that the cold output of the air-conditioning system is matched with heat load, the power which is consumed by the product to produce cold is reduced, and the energy saving and emission reduction of the product is realized.