A water-cooled air conditioner comprising: an indoor unit including a first heat exchanger that performs heat exchange between air and refrigerant, the indoor unit for cooling or heating a space to be air conditioned; an outdoor unit installed separately from the indoor unit, the outdoor unit including at least one compressor (310, 320), a second heat exchanger that performs heat exchange between water and refrigerant, and a main refrigerant valve connected to the first and second heat exchangers to change a flow direction of the refrigerant; a cooling tower installed in a connected manner to the outdoor unit, for cooling water flowing to the second heat exchanger; and a boiler installed in a connected manner to the outdoor unit, for increasing a temperature of the water flowing to the second heat exchanger, characterized in that: the second heat exchanger is a plate-type heat exchanger formed with a plurality of thin plates separated from one another by predetermined gaps that form spaces through which refrigerant and water flow; the second heat exchanger includes an anti-freezing device provided at a side thereof for preventing freezing of water within the second heat exchanger; and the anti-freezing device comprises: a water return temperature sensor sensing the temperature of water that has exchanged heat with refrigerant within the second heat exchanger; a 3-way valve to switch the flow direction of the refrigerant according to the temperature of the water sensed at the water return temperature sensor, the 3-way valve including an intake port connected to the second heat exchanger and a first discharge port connected to the first heat exchanger; and a refrigerant guide pipe coupled to the 3-way valve to guide the flow of refrigerant from the at least one compressor into the 3-way valve. an indoor unit including a first heat exchanger that performs heat exchange between air and refrigerant, the indoor unit for cooling or heating a space to be air conditioned; an outdoor unit installed separately from the indoor unit, the outdoor unit including at least one compressor (310, 320), a second heat exchanger that performs heat exchange between water and refrigerant, and a main refrigerant valve connected to the first and second heat exchangers to change a flow direction of the refrigerant; a cooling tower installed in a connected manner to the outdoor unit, for cooling water flowing to the second heat exchanger; and a boiler installed in a connected manner to the outdoor unit, for increasing a temperature of the water flowing to the second heat exchanger, the second heat exchanger is a plate-type heat exchanger formed with a plurality of thin plates separated from one another by predetermined gaps that form spaces through which refrigerant and water flow; the second heat exchanger includes an anti-freezing device provided at a side thereof for preventing freezing of water within the second heat exchanger; and the anti-freezing device comprises: a water return temperature sensor sensing the temperature of water that has exchanged heat with refrigerant within the second heat exchanger; a 3-way valve to switch the flow direction of the refrigerant according to the temperature of the water sensed at the water return temperature sensor, the 3-way valve including an intake port connected to the second heat exchanger and a first discharge port connected to the first heat exchanger; and a refrigerant guide pipe coupled to the 3-way valve to guide the flow of refrigerant from the at least one compressor into the 3-way valve. a water return temperature sensor sensing the temperature of water that has exchanged heat with refrigerant within the second heat exchanger; a 3-way valve to switch the flow direction of the refrigerant according to the temperature of the water sensed at the water return temperature sensor, the 3-way valve including an intake port connected to the second heat exchanger and a first discharge port connected to the first heat exchanger; and a refrigerant guide pipe coupled to the 3-way valve to guide the flow of refrigerant from the at least one compressor into the 3-way valve.