This paper proposes a fast imaging auto-focus method to adjust the focus of laser diode products. The image of laser diode spot is processed to be a three-value image which is distinguished to be the background, blurred region and bright region. The ratio of areas between blurred and bright regions is used to judge whether the focusing is well-done. A four-point method is modified for this three-value image to get the areas of the blurred and bright regions. Thus, we can get the criterion for auto-focus. In the meantime, we can also get the offset of the laser diode from the center of laser diode spot. The four-point method is faster and more accurate than the area, three-point, contour-tracking and least-squares error methods of center-positioning. This auto-focus method of spatial domain is also faster than other methods of the energy strength in the frequency domain. Finally, we adjust the focus length of laser light by the servo control card and the AC servo motor, and then we can get the optimal focus length from the former image processing.