The invention provides a control method and system for a multi-connected air conditioner under the refrigeration working condition. The multi-connected air conditioner comprises a multi-connected air-conditioning unit and a fresh air unit; the multi-connected air-conditioning unit comprises a plurality of indoor units; and the fresh air unit comprises an indoor heat exchanger and a valve body usedfor controlling a flow rate of a refrigerant which enters the heat exchanger. The control method for the multi-connected air conditioner under the refrigeration working condition comprises the following steps of after receiving a refrigeration turning-on command, acquiring an air return temperature of the fresh air unit; calculating a current superheat degree according to preset calculation requirements corresponding to the air return temperature; and calculating a regulation opening degree of the valve body according to a difference between the current superheat degree and a target superheatdegree, and controlling the valve body according to the regulation opening degree. Aiming to different air return temperatures, the current superheat degrees are calculated according to different calculation requirements, and the opening degree of the valve body is controlled through the difference between the current superheat degree and the target superheat degree, so that the problem that during high-temperature refrigeration or low-temperature refrigeration, the fresh air unit occupies too much or too little refrigerant to influence the refrigeration effect of the multi-connected air conditioner is solved.