The present invention relates to a geothermal system comprising a multitube vertically-sealed underground heat-exchanger and a method for installing same. The geothermal system comprising a multitube vertically-sealed underground heat-exchanger according to the present invention comprises: a geothermal well formed by vertically excavating a foundation; a heat pump which is arranged in the foundation, and which comprises a circulating pump; and a connection tube, an auxiliary facility, and a multitube vertically-sealed underground heat-exchanger which are buried and installed in the geothermal well, and which are connected to the heat pump such that a thermal fluid thermally restored in the geothermal well is supplied to the heat pump through the circulating pump, and the thermal fluid that has undergone heat exchange in the heat pump is recovered back to the geothermal well and thermally restored therein. The multitube vertically-sealed underground heat-exchanger comprises: a water-return head portion arranged in the lower side of the geothermal well; a water-return portion which is connected to the heat pump and the water-return header portion, and which comprises an outflow tube, a water-return connection tube, a distribution tube, and a water-return tube; and a supply portion which is connected to the heat pump and the water-return header portion, and which comprises an inflow tube, a supply/connection tube, a confluent tube, and a supply tube. The water-return header portion forms a boundary of function as the supply tube and the water-return tube are connected to and communicate with each other.