The invention discloses a tri-cogeneration system and a tri-cogeneration method for generating power, producing hydrogen and producing fresh water by the aid of a high-temperature gas-cooled reactor of a nuclear power plant. The tri-cogeneration system comprises a closed helium circulating system of the nuclear power plant, a superheater, a steam generator, a raw seawater pipeline, a flash evaporator, a steam ejector, a seawater desalting device, a water feeding pump, a solid oxide electrolytic tank and an auxiliary steam pipeline. The tri-cogeneration system and the tri-cogeneration method have the advantages that the circulating system of the nuclear power plant, steam electrolysis and hydrogen production processes and distillation seawater desalination processes are coupled with one another, the power is generated by the aid of a helium gas turbine at first, waste heat of partial exhaust gas of the helium gas turbine is used for electrolyzing steam and producing the hydrogen, low-temperature waste heat cooling water of a cooler is used for carrying out distillation seawater desalination by means of coupled flash evaporation and steam injection, and accordingly diversified water, power and hydrogen tri-cogeneration modes can be implemented; the electric power and the fresh water which are required for steam electrolysis and hydrogen production can be provided by means of seawater desalination and nuclear power generation, accordingly, the tri-cogeneration system is high in circulating heat efficiency and good in adjustment performance and has a good economic benefit, the reliability of equipment can be improved, environmental pollution can be reduced, the tri-cogeneration system and the tri-cogeneration method have excellent application prospects, and the like.