A vehicular air conditioning system includes: a heat pump side refrigerant circulation line including a compressor, a water-cooled heat exchanger, a heat pump mode expansion valve, an air-cooled heat exchanger, an air conditioner mode expansion valve and an evaporator, the heat pump side refrigerant circulation line configured to generate cold energy in the evaporator in an air conditioner mode and to generate heat in the water-cooled heat exchanger in a heat pump mode, the air-cooled heat exchanger configured to allow a refrigerant to exchange heat with an ambient air in the air conditioner mode and the heat pump mode; and a heat exchange air switching part configured to switch the type of the air heat-exchanged in the air-cooled heat exchanger depending on the air conditioner mode or the heat pump mode.