We proposed a CPMF (combinative probability mass function) model for universal steganalysis to images from nature images and forged images, which can present statistic divergence between cover or forged image and steg image prominently. Based on the two features of images and CPMF mode, we discussed the affection of steg process and forged process on the nature images. This model relied most on the statistic relativity of between two color channels. At last, many experiment results are given to support our theory.