In this thesis, we investigate the optical fixing in photorefractive crystals. There is a periodic distribution of refractive constant in crystals by using two different wavelengths recording, to keep the signals storage in the crystal for a long time. Base on Kukhtarev equation, we use a single-carrier band-transport model with two energy levels and obtain the changes of the space charge field with time. Therefor we are able to simulate the recording and fixing processes. We select two crystals, Bi12SiO20 and Bi12TiO20 and use their parameters for simulation respectively. Then we discuss how material parameters affect the recording and fixing processes. Finally we set up an experiment to stand for the simulation and compare the difference with these two crystals.