The change of the fiber Bragg grating (FBG) spectrum induced by intense light is caused by the nonlinear effect together with the photo-thermal effect. Based on the sensing model of FBG, the contribution of nonlinear effect and the photo-thermal effect on the FBG spectrum change is analyzed by using the nonlinear refractive index coefficient and one-dimensional steady state heat equation. By analyzing the relationship between the wavelength shift and phase change, nonlinear refractive index coefficient calculation model is built. And the photo-thermal effect on the spectrum chirp and wavelength shift is confirmed by theoretical simulation of temperature distribution. As the theoretical and experimental results show, for conventional single mode fiber, FBG spectrum change is mainly caused by the photo-thermal effect rather than the nonlinear effect when a relatively lower power pump is injected to single mode optical fiber.