Refractive distribution in fiber Bragg grating(FBG) is expanded by Fourier series. The relation between step index profile and sinusoidal distribution is analyzed, and it is reasonable for using step index profile distribution to do the calculation. Rouard numeric method is used to analyze transmission spectrum of the cascaded Fabry-Pérot (F-P) cavities with length about sub-micron level. Based on two-beam optical interference approach, the characteristics of the cascaded fiber cavity and the connection between F-P cavity and FBG are given. An advisable method is proposed in which FBG can result from the cascade of fiber cavities, and basic principle of the FBG is established by utilizing the cascaded F-P cavity approach.