An air conditioning system and a control method thereof. The air conditioning system includes: a plurality of indoor units connected in parallel; a plurality of outdoor units connected in parallel; and a coolant circulation circuit which circulates coolant through each of the indoor units connected in parallel and each of the outdoor units connected in parallel respectively, and which exchanges heat with each of the indoor units and each of the outdoor units; wherein the air conditioning system further includes a controller which, based on a number of actively operating indoor units, a total number of the indoor units and a total number of the outdoor units, defines an upper limit of a number of actively operating outdoor units, so that the flow rate of the coolant flowing through the actively operating outdoor units is not lower than a preset flow rate.