Dynamic equations applicable to the acousto-optic bistable states arepresented, from which a criterion of bistability with multi-order and multi-parameter isdeduced. It has been shown that a traveling wave Raman-Nath acousto-optic bistablesystem with the first order diffraction as the feedback exhibits bistability and hysteresisin multi-order diffractions whatever the input parameter is light intensity or biasingvoltage.