The invention discloses an air-conditioning system and a control method for the same. The air-conditioning system comprises a plurality of outdoor units, a plurality of indoor units connected with the plurality of outdoor units through a liquid pipe, a low-pressure gas pipe and a high-pressure gas pipe separately, a plurality of expansion valves in one-to-one correspondence with the heat exchangers of the plurality of outdoor units, and serially connected between the liquid pipe and the plurality of outdoor units, and a controller adjusting the opening degrees of the corresponding expansion valves taking the heat exchangers as evaporators, of the outdoor units, in the plurality of outdoor units according to the outlet temperatures of the heat exchangers of the refrigeration indoor units, and adjusting the corresponding expansion valves taking the heat exchangers as condensers, of the outdoor units, in the plurality of outdoor units according to the middle-part temperatures of the heat exchangers of the heating indoor units after the air-conditioning system enters a mixed mode. Through the accurate adjustment of the air-conditioning system disclosed by the invention for the opening degrees of the electronic expansion valves of the outdoor units, the refrigeration and heating capacity of the mixed mode under a low-temperature environment of the outdoor units can be improved, and meanwhile, the enough superheat degree of a compressor is ensured and the system reliability is improved.