In this paper the distortion of image collimating system is analyzed,and then an on-line self-calibration method with double-frequency laser length measuring result as length datum is presented. In this method,the stripe on grade rod is taken as calibrating and measuring object ,many stripe edge positions of image and corresponding length are obtained, and a model mathematical between them is established. Then according to the model, the distortion error of collimating system is rectifled. Experiment result shows that the rectification method can reduce one dimension error to about 15 % of pre-rectification, and the aided equipment and standard calibration board are needless,so the method is simple and easy to carry out.