An optical model of side polished fiber Bragg grating was proposed in this paper. The shift of grating resonance wavelength and the reflectance variation of grating spectrum were calculated with changing the residual cladding thickness and refractive index of the materials overlaid the side polished area. The calculated results and experiment showed that if the reflectance index(RI)of coating materials is closer to the fiber core RI,and the residual cladding thickness is less than 3μm,the obvious variation of grating resonance wavelength and grating spectrum will be caused by the slight changes of coating materials′ RI.