With the development of laser technology and quantum communication technology, an acousto-optic modulator has become an important device in many fields. The diffraction efficiency and polarization maintaining ability are important indexes of an acousto-optic modulator. In this paper, through the theoretical and experimental investigation of the diffraction efficiency of an acousto-optic modulator, we propose a theoretical method to improve the acousto-optic diffraction efficiency and design a positive feedback acousto-optic modulation system, which can make the polarization of any arbitrarily polarized light maintained. The experimental results show that this acousto-optic modulation system can significantly enhance the diffraction efficiency of the acousto-optic modulator and guarantee the polarization maintaining of any arbitrarily polarized light during propagation in the field of quantum communications. The research results provide a reference for the research of optical fiber coupled acousto-optic modulators and quantum devices based on acousto-optic modulators.