The invention discloses a water seal device for a nuclear power plant voltage stabilizer and the nuclear power plant voltage stabilizer. The water seal device comprises a voltage stabilizer upper seal head (1), a voltage stabilizer safety valve adapter tube (2) inserted into the voltage stabilizer upper seal head (1), a safety valve process pipe (3) used for connecting a voltage stabilizer safety valve with the voltage stabilizer safety valve adapter tube (2) as well as a water collecting cover (4) which is arranged on the inner side surface of the voltage stabilizer upper seal head (1) and covers a port in one end of the voltage stabilizer safety valve adapter tube (2). The water seal device can perform water seal on the port in one end, communicating with inner vapor phase of the voltage stabilizer, of the voltage stabilizer safety valve adapter tube, so that hydrogen in the inner vapor phase of the voltage stabilizer cannot leak through the voltage stabilizer safety valve; furthermore, the water collecting cover is arranged in the voltage stabilizer, layout space outside the voltage stabilizer is saved, and besides, with the adoption of high-temperature water seal, hydraulic load borne by a downstream discharge pipeline after the voltage stabilizer safety valve is switched on can be effectively reduced.