The mode coupling in multimode fiber Bragg gratings (MMFBGs) is analyzed by using the coupled-mode theory. The reflection spectra of MMFBGs excited by one mode and two modes are simulated. The results of the calculation show that MMFBGs have multiple reflection peaks due to the coupling between the same modes in counter-propagating direction and the coupling between the adjacent modes in counter-propagating direction, and the spectra depend on excitation conditions of the bounded modes, such as mode power and mode number.