Abstract: The nonuniformity was a major concern when X-ray images were evaluated, and it would affect both the detection and classification ability of the imaging instrument. The theoretical principles of the cause for the nonuniformity of digital image were analyzed, linear photoelectric response model of each pixel channel in the imaging instrument was presented, and a multipoint linear fitting correction algorithm was proposed. The images and the standard deviations for the images were given before and after the correction. The nonuniformity correction method was useful for the improvment of image quality when linear relationship between image pixel output and X-ray tube current existed.