Abstract: The electric field of the transmission mode in an optical fiber waveguide is analyzed theoretically. The discipline describing the influence of optical fiber refractive index changes on waveguide electromagnetic field distribution is obtained. A real time experimental measurement system of refractive index changes has been built by using bulk fused silica material and fiber Bragg grating under 60Co -ray radiation. The refractive index changes of two candidates with given radiation doses and mode field of fiber are measured in the experiments respectively. The results show that the refractive index of the optical fiber increases with the radiation dose; the refractive index changes cause the variations of the field intensity distribution in the transmission mode, which results in radiation-induced waveguide losses; within a certain dose range of 0-2000 Gy, the optical fiber meets the weakly guiding boundary conditions and maintains the confining ability on the transmission mode.