The　longitudinal　electro-optic　effect　and　the　multi-dimensional　isotropic　acousto-optic　effect　in　square　system　and　cubic　system　are　researched.　KDP　of　the　square　system　and　GaP　of　the　cubic　system　are　determined　as　optimum　crystals　for　multi-dimensional　isotropic　acousto-electro-optic　devices.　Through　calculating　the　function　characters　of　acousto-optic　interaction　in　the　two　crystals,　some　optimum　operating　modes　for　its　two-dimensional,　three-dimensional　and　four-dimensional　isotropic　acousto-electro-optic　effect　are　determined.　Multidimensional　acousto-electro-optic　devices　have　applications　in　multi-channel　optic　communication　and　optic　signal　processing.