Light ray changes can always alter the image data block structure, which leads to matching and recognition errors in the image. This paper analyzes how the intensity of light impacts on the gray of ground targets, and proposes a kind of method to recognize the target based on data block as well as dividing the target into inverse color or direct color part. We used the Nprod matching and character distinguishing method to extract the target to avoid the image gray disturbing by the ray. It is proved that the method we put up with can overcome the impact of changes in light and recognize different kinds of targets aground well.