A theoretical model, in which the band transport equations and the coupled wave equations are both considered, to describe the characters of recording and thermal fixing in LiNbO3 is presented. It is found that in simultaneous-fixing, the existing ionic grating affect the writing of the electronics grating by reducing the coupling gain and the grating envelope of the fixed index grating by simultaneous fixing is quite uniform inside the photorefractive crystal with comparison to by post-fixing. The resulting diffraction efficiency of the fixed volume grating is mainly dependent on the initial intensity modulation of the two writing beams. The model also shows that fixing in a high doped material, at its developing stage, the electric grating can not be erased which resulting in low diffraction efficiency. A set of experimental demonstration are presented.