The Fourier mode coupling theory is applied to the analysis of the fiber Bragg grating based all-fiber acousto-optic modulator for the first time. Compared with the existing analysis methods, the algorithm of this model is simple and easy, and transmission characteristics of the modulator can be acquired effectively and efficiently. Based on the theory, the performances of the modulator, related to ultrasonic frequency and amplitude of acoustically induced strain, are investigated. Simulation results show that in reflection spectra of the modulator, the wavelength interval between the primary relection peak and the secondary relection peak is proportional to ultrasonic frequency, and the reflectivity of the reflection peak varies periodically with intensity change of the amplitude of acoustically induced strain. In addition, with the same amplitude of acoustically induced strain, more secondary reflections exist in the low-frequency ultrasonic modulated fiber Bragg grating, and the periodic variation of the energy reflected by fiber Bragg graing is more obvious. In the experiment, the fiber Bragg graing is modulated by an ultrasonic wave with a frequency of 885.5 KHz. The experimental results accord well with the simulation results.