The interactions between X-ray and the sample and crystals were investigated, and the influence of small angle scattering on images was emphatically analyzed in diffraction enhanced imaging in this paper. Not only were the additional noise term caused by small angle scattering introduced, and more general DEI equation established, but also expressions of absorption contrast, extinction contrast and refraction contrast were given for the peak image and the middle slope image. With the new DEI equation, the contrasts of two kinds of resultant images (apparent absorption image and refraction image) were discussed.