Ship seawater system drainage device and drainage method thereof

The invention relates to the technical field of ships, and discloses a ship seawater system drainage device and a drainage method thereof. The ship seawater system drainage device comprises a working header pipe, a discharge pipe and a gas supply pipe, the working header pipe comprises a water inlet and a water outlet, the water inlet is communicated with a sea chest, the water outlet is communicated with a ship water device, a water inlet section is arranged between the water inlet and a designed waterline, and a water using section is arranged between the designed waterline and the water outlet; one end of the discharge pipe is communicated with the water inlet section, the other end of the discharge pipe is communicated with the sea chest through a discharge valve, and a before-pump isolating valve is arranged between the water inlet and the discharge pipe; one end of the gas supply pipe is communicated with the water section, the other end is communicated with the gas supply device through an injection va