Starting from the mechanism of intensity-dependent absorption found experimentally in BSO crystals, we derived-the nonlinear coupled-wave equations, taking the interaction between self-diffraction and recording of grating into account. They are used for describing the two-wave interaction in BSO crystals. With the model for dynamic interaction, a method of numerical solution was put forwwrd. The calculated results accounted successfully for the disagresment between previous theories and experiments. Under various physical conditions, the characteristics of the two-wave inter action vrere calculated. They were born out by experiments.