The utility model relates to an underground heat exchanger in a ground-source heat-pump unit, which belongs to the technical field of ground-source heat-pump air conditioners and is characterized in that a refrigerant pipe and an arranged conductive flat plate are combined into a whole, and a contact area between the underground heat exchanger and soil is greatly enlarged due to the arrangement of the conductive flat plate so as to effectively improve the heat exchange effect, reduce the construction cost and the workload of the underground heat exchanger and improve the construction efficiency. The utility model shortens the length of the refrigerant pipe by more than 20 percent than an underground U-shaped pipe heat exchanger in the prior ground-source heat-pump unit so as to reduce the system resistance, enlarges the contact area with the soil by approximate 10 times than the original underground U-shaped pipe heat exchanger so as to obviously improve the heat exchange effect, is matched with a host in a ground-source heat-pump unit of a direct expansion type and can be widely applied to suburbs and rural families for refrigeration and cooling in summer, heating in winter and living hot water supply all year round so that the unit has better underground heat exchange effect and lower cost.