The present invention relates to an air-conditioner system for a vehicle and a method for controlling the same, and more particularly, to an air-conditioner system for a vehicle and a method for controlling the same which can change a target superheat degree depending on variation in an amount of refrigerant discharged from a compressor, namely, gradually decrease the target superheat degree by stages as the amount of refrigerant discharged from the compressor decreases, and control an electronic expansion valve based on the target superheat degree dropping by stages, thereby preventing performance degradation of an air conditioner and stabilizing the system by restraining a change in the superheat degree in an area where an amount of refrigerant discharged from a compressor varies.