A water-cooled air conditioner is provided, and includes an indoor unit, an outdoor unit, a cooling tower, and a boiler. The indoor unit cools or heats a space, and includes a first heat exchanger that performs heat exchange between air and refrigerant. The outdoor unit is installed separately from the indoor unit and includes a second heat exchanger that performs heat exchange between water and refrigerant. The cooling tower is connected to the outdoor unit, and cools water flowing to the second heat exchanger. The boiler is connected to the outdoor unit, and increases a temperature of the water flowing to the second heat exchanger. The second heat exchanger is formed with a plurality of thin plates separated from one another by predetermined gaps through which refrigerant and water flow. The second heat exchanger includes an anti-freezing device that prevents freezing of water within the second heat exchanger.