The initial phase of the two fiber Bragg gratings used in FabryPérot cavity was considered and the reflectivity and transmissivity expressions of fiber grating-based FabryPérot cavity at any FBG’s initial phase were derivated by coupledwave equation. The phase factors in the expression include cavity length modulated by propagation constant, phase mutation of the fiber Bragg grating, initial phase of the two fiber Bragg gratings and fiber grating length modulated by space frequency of the fiber Bragg grating. The influence of the latter two factors on reflection spectrum was analyzed, and a method to regard a fiber grating-based Fabrypérot cavity as ideal FabryPérot cavity was presented.