In this paper,the CdS films were prepared by Ultra high vacuum evaporation on the glass substrate.The different preparation conditions of different air pressure,substrate temperature,and Annealing treatment are be taken into account in this paper.All of the Samples are the wurtzite structure.The structure of CdS poly-crystal films are rather sensitive for the air pressure especially at the pressure near 2.4×10-2pa,the substrate temperature as well.Crystal structure of films is measured by energy-dispersive X ray diffraction.