A　piezoelectric　strength　acousto-optic　(AO)　coefficient　of　piezoelectric　crystals　is　introduced.　It　is　a　result　of　influences　of　AO　effect,　electro-optic　(EO)　effect　and　piezoelectric　effect　at　the　same　time.　A　coupled　wave　equation　of　surface-acoustic-wave　(SAW)　AO　effect　is　given　from　basic　equations　of　the　parameter　interaction.　A　diffraction　efficiency　formula　of　the　SAW　AO　effect　is　given.　The　formula　indicates　that　under　the　condition　of　weak　AO　effect　the　intensity　of　diffractive　guided　optic　wave　is　directly　proportional　to　ultrasonic　power.　So　the　SAW　AO　device　can　be　used　as　modulator.　The　SAW　AO　device　has　many　advantages,　of　small　volume,　good　stability,　low　energy　consumption　and　it　is　easy　to　integrate.　The　device　has　applications　in　optic　communication　and　optic　signal　processing.