The sealing state of the watertight door (SSWD) is important information for the safe operation of a ship. A monitoring method for the SSWD based on fiber Bragg grating (FBG) sensor is designed, which can monitor the opening and closing state of the watertight door in real-time. A highly sensitive and temperature-independent sensor is installed under the rubber of the watertight door, the relationship between the displacement of the door panel and the wavelength shift of FBG is linearly fitted with the sensitivity of about 314.511  pm  /  mm, and the error percentage with the finite element analysis is 6.22%. The watertight door is opened and closed many times, the results show that the sensor has good stability and reliability, and the opening state, closing state, and the degree of opening and closing can be accurately judged by the wavelength shift of FBG. At the same time, the sensor can also identify the foreign body within a certain range on the rubber surface.