Bi12SiO20(BSO) is a kind of photosensitive material. Its resistance would drop sharply when exposed to the short-wave-length light, such as UV, so it would be used as the sensitizing layer in the liquid crystal spatial light modulator (LC-SLM). In this paper, we have theoretically studied the principle of LC-SLM and the photoconductivity of BSO. The relationship between photocurrent and the power of the illumination is introduced. In the experiment, the photoconductive effect of a slice of BSO with different wave-lengths illumination is measured. The optical path difference caused by the LC-SLM exposed to the UV is calculated by computing the distribution of the liquid crystal director. The change of the optical path difference caused by UV is similar to the change of optical path difference when the LC cell is applied different voltages in the experiment;