Refractive index changes of hydrogen loaded fiber Bragg grating induced by multiple post-exposure are researched.Changes of fiber grating refractive index induced by multiple post-exposure are analyzed based on the two-step process photosensitive model.The relationship between grating refractive index and multiple postexposure is obtained under different initial refractive index distribution,and the mathematical models about mean refractive index and modulation refractive index of fiber grating are built up.Furthermore,variation characteristics of fiber Bragg grating wavelength and refractive index under multiple post-exposure are experimentally studied using 193 nm UV laser.The results show that,under multiple post-exposure,the grating wavelength gets red shifted and the shift quantity is related to post-exposure conditions.The grating refractive index increases firstly and then tends to be stable.The growth rate is related to post-exposure conditions and the maximum value is determined by the initial refractive index distribution of fiber Bragg grating.