A method of preparing CdS photoconductive film used in AC Liquid Crystal Light Valve (LCLV)is developed. It employs general evaporating techniques in the vacuum system. After evaprating, the, CdS film is annealed at high temperature and diffused with proper Cu impurity as acceptor. Thus a CdS film with smooth surface, high dark resistance and well photocondactivity is obtained. It has been known by the measurement that the dark resistance is higher then 109Ω.cm. The ratio of light to dark electrical current is of the order of 10s, the response time ia shorter than 100 ms.