The multifrequenoy acoustooptio coupled mode equations, including mormal and abnormal aooustooptio interactions, are presented in this paper. The solutions of the equations for two independent acoustic signals in the Bragg limit are derious linear and nonlinear effects of the multifrequency acoustooptio interactions are analysed theretically. These include diffraction efficiency, compression, cross modulation, and additional intermodulation intensities. Specially the differences between the abnormal diffraction and the normal diffraction are pointed out and analyse. The experimental results are in good agreement with theoretical analyses.