New equations diseribing formation of space charge field are derived by taking self-diffraction and nonlinear absorption into account. It is shown that the phase mismatch of space charge field with light intensity grating plays very important role in photorefractive effect. A equation disoribing the incline and bent of eqniphase surfaces is also derived. By using/numeric al oaoulations we find the role of energy transfer in BSO. Some material parameter will affect the speed of the process, which shows the possibility of using CW laser in studying the response to temporal process.