Abstract: Some equations of anisotropic acousto-optic (AO) diffraction geometry have been derived by the aid of tangent condition. The relationships of incident angle of optic wave vector, diffraction angle of optic wave vector, acoustic extremum frequency and AO merit vs acoustic deviation angle in z-t plane are given in detail. These equations are more simple and accurate than traditional Dixon equations. The calculation results indicate that the center frequencies of the TeO₂ anisotropic AO devices have a minimum threshold, and the mode of the +1 order diffraction is better than -1 order to same center frequency. For the convenience of the device design, some design parameters and function parameters are showed for the TeO₂ anisotropic AO device below the center frequency of 200 MHz.