The limit method, balance method and transfer matrix method for fiber Bragg grating (FBG) Fabry-Pérot (F-P) cavity are studied and proved uniform. The influence of the reflection phase of fiber Bragg grating complex amplitude transfer matrix on the transmission of fiber Bragg grating Fabry-Pérot cavity is emphasized. The Rouard method, which is capable of calculating fiber Bragg grating with arbitrary refractive index modulation, is used to calculate fiber Bragg grating Fabry-Pérot cavity and modified by introducing initial phase of refractive index distribution to calculate the reflection phase precisely. The transmission spectrum of fiber Bragg grating Fabry-Pérot cavity with multi-peak structure is obtained with the above results and the calculation results are proved by experiments.