Abstract: A fiber coupled-mode equation with perturbation is given from basic equation of the parameter interaction. Considering ultrasonic wave as perturbation, a coupled-wave equation group of fiber acousto-optic interaction is deduced. This equation group includes equations of forward wave and backward wave. The equation group is a theory base of fiber acousto-optic effect. Through solving the equation group, a formula of backward wave efficiency is obtained. It is proved that under condition of weak acousto-optic interaction, the intensity of the backward wave is directly proportional to acoustic power approximately. The research in this paper will lay theoretical foundation on manufacture of the fiber acousto-optic intensity modulator. The new modulator can be widely applied to optical fiber communication.