The characteristics of transmission spectrum of the Fabry-Perot (F-P) cavity composed of two fiber Bragg gratings (FBG) are analyzed detailedly in this paper. The influencing factors of the position of the longitudinal mode are pointedout. The relationship and difference between the FBG F-P cavity and normal F-P cavity are discussed. The phase factor of the amplitude reflection coefficient in the range of the FBG main lobe is simulated linearly using the effective mirror surface model. The effective length of the FBG is defined and the expression of the effective length is obtained. Furthermore， the effective length of the FBG is brought into the equivalent cavity length of the FBG F-P cavity. The interval between longitudinal modes of the FBG F-P cavity is calculated using the equivalent cavity length. Both the numerical simulation and experimental results show that the interval calculated using the equivalent cavity length accords well with the true interval between longitudinal modes of the FBG F-P cavity.