The invention belongs to the technical field of heat pump units, and particularly provides a heat pump system and a defrosting control method thereof. The problem that when an existing heat pump system is used for defrosting, the influences on the temperature at the heat exchange tail end are large is solved. The heat pump system comprises a first heat exchanger, a heat exchange tail end, a waterinlet pipe and a water outlet pipe, the water inlet pipe and the water outlet pipe are arranged between the first heat exchanger and the heat exchange tail end, the water outlet pipe is provided witha first electromagnetic valve and an electric auxiliary device, a bypass pipe is arranged between the water inlet pipe and the water outlet pipe, and a second electromagnetic valve is arranged on thebypass pipe. The defrosting control method comprises the steps that the first electromagnetic valve is closed, and the second electromagnetic valve is opened; and the electric auxiliary device is started, and an air conditioner unit enters a defrosting mode. By means of the arrangement, influences of the first heat exchanger on the temperature of the heat exchange tail end are reduced, the indoorheat comfort of a user is improved, and the defrosting process is accelerated.