In view of the rapid development of the modern science and technology, image processing technology is also widely used in various fields. More and more digital images are tampered, which brings a lot of difficulties and troubles. This paper analyzes the shadows of objects which are produced by the point light source or the line light source, and makes a preliminary judgment by calculating the grey level. Based on Lambert illumination model and optical properties of consistency, this paper proposes a new technique to judge whether an image is tampered or not. At last, the experimental results are obtained by analyzing the shadows of a lot of images.