The invention relates to a heat exchanger of a water tank of a heat pump and a heat exchange method thereof in the field of water heaters. The heat exchanger comprises a heat exchanger for performing the heat exchange for a medium and water in a liner of the water tank, wherein the heat exchanger is provided with an inlet and an outlet connected with a heating device. The inlet and the outlet are respectively connected with a reversing valve; and the reversing valve is connected with a medium inlet and a medium outlet of the heating device, and is connected with a control unit. The medium flows into the heat exchanger after being heated by the heating device, is exchanged heat with the water in the water tank by the heat exchanger, and then flows back to the heating device for circulation. When reversing conditions are satisfied, the flowing direction of the medium in the heat exchanger is changed under the effect of the control unit. According to the device and the method, the heated medium can be changed the flowing direction in the heat exchanger so as to realize the purpose of uniform heating of the water in the water tank. Meanwhile, the device and the method also improve the heat exchange efficiency, and shorten the heating time.