Abstract: Discrimination,reflectivity and FSW(free spectrum width),which are determined by the geometric attenuation factor of optic field and reflectivity of two ends greatly,are the important factors of FFP(fiber Fabry-Perot).Because of the imprecision of the old attenuation factor,which is based on identical beam model,a new model based on Gaussian beams is provided,and the equation of which is found.Then the influence of cavity length and refrectivity of ends to discrimination and reflectivity of FFP is analysed deeply. Conclusively,a FFP wavelength demodulator is designed for optic fiber grating.