It was important to identify Point Spread Function (PSF) accurately in blind motion-blurred image restoration. When we knew motion-blurred direction, the two-dimensional PSF problem could be transformed into one-dimensional problem. We used the first order difference auto-correlation method to identify motion-blurred size. This method was proved feasibly in theory. Then we researched and deduced the influence of noise particularly. In order to tackle the noise problem, the improved method was put forward. Experiment results on simulated images show accuracy to identify size. And our improved method enhances robustness to noise. At last, using proposed method to identify the size of the real motion-blurred image, the distinct restored image also validates the usability and validity of the proposed method.