The invention relates to an underground transformer substation for an outdoor power supply system. The underground transformer substation comprises a box which is arranged underground, wherein a protecting cover plate is arranged above the box. The underground transformer substation is characterized in that a middle interlayer is arranged in a box wall of the box; a semiconductor refrigeration chip is arranged in the middle interlayer, and a refrigerant fluid is filled in the middle interlayer; a radiating fin is arranged on a side wall of the box; the interior of the radiating fin is communicated with the middle interlayer in the box wall; the semiconductor refrigeration chip is connected with a power supply through a first electromagnetic relay; and a temperature sensor is arranged in the box.