The invention discloses a low-voltage preassembled transformer substation, and particularly relates to the technical field of transformer substations, the low-voltage preassembled transformer substation comprises a transformer substation, the left and right side surfaces of the transformer substation are provided with heat dissipation windows, and one end of each heat dissipation window is fixedly connected with the outer wall of the transformer substation. When the rainfall is large, the weight of the water containing box body is increased after much rainwater is accumulated, the lining baffle is pulled to ascend, then the heat dissipation window can be automatically closed, meanwhile, the lower portion of the water containing box body can synchronously drive the fan to rotate to generate certain wind power, and an air barrier is formed on the outer side of the heat dissipation window to conduct multi-stage rainproof protection on the transformer substation. Secondly, water accumulated in the water containing box body is circularly pumped through cooperation of a small water pump and a heat conduction pipe, heat dissipation can be conducted on the inner interlayer, the waterproof protection performance of the transformer substation can be improved in rainy days, normal heat dissipation of the transformer substation in the daily state is guaranteed, and the collected rainwater can be utilized to fully dissipate heat of the transformer substation, so that a good guarantee is provided for the transformer substation.