The utility model provides a water seal pipe automatic water supplementing system based on a thermal power plant exhaust device. The water seal pipe automatic water supplementing system comprises theexhaust device, a vacuum breaking valve, a water seal pipe, a magnetic flap water level gauge, water supplementing equipment and a controller. Wherein the vacuum breaking valve is arranged on a vacuumpipeline connected with the exhaust device, the water seal pipe is arranged at an outlet of the vacuum breaking valve, the magnetic flap water level gauge and the water replenishing equipment are connected with the water seal pipe, and the controller is respectively connected with the magnetic flap water level gauge and the water replenishing equipment. The water supplementing device comprises awater supplementing pipeline and a water supplementing electromagnetic valve arranged on the water supplementing pipeline. The water supplementing device further comprises a water supplementing manualdoor. The magnetic flap water level gauge comprises a magnetic floater, an indicator, a liquid level high switch and a liquid level low switch, the system can automatically supplement water to the water seal pipe of the exhaust device, so that the safe operation of the exhaust device of the thermal power plant is ensured, and the system has definite significance for energy conservation and consumption reduction of the thermal power plant.