The invention discloses an underground hydroelectric power generation system for plain regions. Eastern coastal areas in China have rich rain water all the year round and are rich in surface water reserves. For meeting the demands of people for water in long term life and production, underground water is exploited so excessively as to lead to water exhaustion and ground settlement, to endanger ecological harmony and to urgently await active recharge supplement. In plain regions where water and electric resources lack originally, the implementation of underground hydroelectric power generationrepresents innovation and creation in fields of renewable energy sources and contributes to the restoration of underground water resource balance and the improvement on ecological environment and lifequality of people, thereby achieving a plurality of effects. A water well which has a diameter of 24 centimeters and a depth of more than 60 meters is dug on the ground and runs through an underground water supply layer (8) and a water drainage layer (12). An upper well pipe (6), a lower well pipe (11), a water intake pipe (7), a water drainage pipe (15), an intercepter plate (10), a check valve(9), a flow regulating valve (4), a vacuum hydraulic permanent magnet generator (19) and an electrical modulation box (20) are arranged. The system can be normally operated only when being tightly sealed.