The invention aims to provide a heat pump system. The problem of a low utilization rate of a high-temperature heat source of an existing heat pump system is mainly solved. The heat pump system provided by the invention is provided with two refrigerant circulation loops. Refrigerant flowing out of a condenser in one refrigerant circulation loop and refrigerant, which is about to flow into an inlet of an evaporator, of the other refrigerant circulation loop are subjected to heat exchange in a middle heat exchange device. The evaporating temperatures of the two refrigerant circulation loops are different and the condensing temperatures of the two refrigerant circulation loops are also different by arranging the middle heat exchange device. The heat pump system can utilize a high-temperature part of the high-temperature heat source and also utilize a medium-low temperature part of the high-temperature heat source by means of the different evaporating temperatures, so that the utilization rate of the high-temperature heat source is greatly increased. Besides, a gradient heating effect on a heated medium can be achieved by means of the different condensing temperatures, so that the working temperature difference of the condensers is reduced and the heating performance coefficient of the heat pump system is improved.