The ionizing radiation effects on fiber Bragg grating are theoretically analyzed based on the color center model. And the relationship between the radiation induced refractive index change and dose is deduced. Fiber Bragg grating sample are irradiated by ray with a total dose of 1 106 rad. Under radiation the reflected spectrum peak wavelength of FBG sample is red shifted, but the full width at half maximent and the reflectivity are not changed obviously. The experiment results accord well with our function. With the low level radiation experiment, this expression can screen the FBG samples and predict their performances under high level radiation.