Aim ming at the process of the Chang'e 3 lunar exploration project, this paper, mainly studies to select the object region in the process of Chang'e 3 soft landing. According to characteristics that pictures photoed in different areas have different sizes of the numerical pixel, it will be established a suitable space rectangular coordinate system and find out a relatively flat areas by construting and calculating flat coefficient. and select the best landing point of Chang'e, and conclude a corresponding trajectory equation under the principle of the minimum path.