In this study, a fiber Bragg grating (FBG) sensor system was designed and attached to an underwater pressure vessel. The pressure vessel was then subjected to various diving depths of up to 100 m to determine the sensor system's ability to correctly identify variations in pressure. A total of four FBG sensors were attached to the loader at angles of 0, 90, 180, and 270°. The results for each of the four FBG sensors were compared and verified by the finite element method. The experimental results showed that when pressure was imposed on the cylindrical structure, the wavelength variations of the four FBG sensors were 130, 190, 190, and 200 με, respectively. The maximum sensitivity of the FBG pressure sensor is −22.364 με/bar. The results indicate that the FBG sensor system can provide accurate measurements of the structural strength of an underwater pressure vessel. In addition, the monitoring technology can be established through comparison and verification with finite element method data. The spectral variations of the proposed optical fiber pressure sensor show good linear relationship with increasing level of strain due to water pressure.