The pattern painting is an important method to protect the target, the pattern painting design is based on the spot shape design. First, the visible spots are extracted automatically from the background image. Then, the statistic characteristics of the girth, area, elongation ratio, rectangular ratio, slightness ratio and dispersing ratio for the background spot are analyzed. Finally, taking the length, rectangular ratio, slightness ratio and dispersing ratio as the spot characteristic vector, the extracted background spot is analyzed by using HCM cluster analysis, and the characteristic parameter is gained. This method can gain the statistic characteristics of the large sample background images, which is useful to for the design of the automatic and quantitative pattern painting.