We theoretically study the optimum design of anisotropic acousto-optic modulator (AOM) based on lithium niobate crystal. Four different kinds of operating modes in the XOZ and YOZ acousto-optic (AO) planes are systematically analyzed by the tangent condition theory and the optimized operating mode is determined. Furthermore, the dependence of the AO merit on the operating frequency and the off-axis angle of the AOM are also obtained by numerical simulations.