The grating theory of acousto-optic effect grating is studied. The relationship between momentum matching and diffraction efficiency is analyzed. The noise reduction mechanism of balance coherent detection is discussed. The system is designed and the relative experiment is test. Noise reduction technology based on acousto-optic effect in coherent balanced detection is verified. The result shows that in the interaction of the sound and light ,a stable grating could be established when the phase of signal is determined, and its diffraction efficiency is bigger than noise signal whose phase is random. By using balance detection, the influence of noise fluctuation to the system output is effective inhibited, and the signal noise ratio is improved.