A transboundary-area water-body monitoring and early warning system includes a monitoring device placed in water body to monitor the water body. The monitoring device comprises a housing body. A control mechanism, a communication mechanism, a sensor and a power box are arranged within the housing body. Power ends of the control mechanism, of the sensor, and of the communication mechanism are electrically connected to an output end of the power box. Control terminals of the communication mechanism and of the sensor are electrically connected to a control terminal of the control mechanism. The control mechanism is configured to receive a signal sent by the sensor, and send the signal from the sensor to a background computer through the communication mechanism. The housing body comprises a lower housing and an upper housing. The lower housing is sealably connected to the upper housing.