Abstract: Because of the fiber sensor′s excellent characteristics, such as small volume, simply structure, high sensitivity, low loss, resistance to electromagnet interference and corrosion resistance, it is used in a microphone to reduce the size of the microphone, and improve its sensitivity and electromagnetic immunity performance. A new design for a reflective fiber microphone is proposed. The Y type reflective multimode fiber sensing probe is adopted. The block diagram of the fiber microphone system is given. Based on the theory of the optic field distribution of the exit light from a fiber, the relationship between the offcentering quantity h and the optical intensity modulate function I(Z″) is presented for the deformed film in sound field.