In the high-temperature gas-cooled reactor of nuclear power plant, pressure sensors play a significant role in the real-time monitoring of the pressure of nuclear system, which ensure the safe operation of the whole system. Fiber Bragg Grating (FBG) pressure sensors are widely used for it advantages of high sensitivity, immunity to electromagnetic interference and high accuracy. However, long-term operation in a high-humidity environment will shorten the lifespan of the sensor and accelerate its aging process, bringing potential risks of operational accidents. In this paper, the main progress during the last several years in the influence of humidity on FBG sensors is reviewed. The basic structure and working principle of FBG pressure sensors are first given. By analyzing previous studies, various physical and chemical processes in which humidity affects the performance of FBG sensors are then elaborated. Humidity causes the dissolution or migration of additives and impurities, which affects the optical performance by changing the refractive index, grating period and stability. Moreover, the elastomer and packaging of the sensors are mostly metal materials or organic polymer materials, so high-humidity environment may also cause corrosion of metal elastomers and moisture absorption expansion or structural changes of polymer elastomers. This review mainly summarize the existing research results and analyze the possible degradation mechanism, which can provide reference for the establishment of FBG pressure sensor’s degradation model and lifespan prediction under high humidity environment.