This paper introduces the formation of optical thin film by the double source electron beam evaporation,the Si/SiO_2 mixed thin film is evaporated on the K9 glass.The reflectivity index of the film is adjustable and the it is higher than that of the Si thin films.The Si thin film which is evaporated by the traditional ways and the Si/SiO_2 mixed thin film evaporated by the double source electron beam have been characterized by atomic force microscopy(AFM).It is found that the fabric of the first one with large holes is loose,and the structure of the last one is compact.The reasons are discussed on the theory of the thin film shape.