In order to realize the automation and digitization of various optical measurements on the optical bench, the LCOS micro-display is used as the target generator and added to the optical bench measurement system to replace the traditional reticle, and the CMOS camera is used as the image receiver to construct the digital optical bench optical measurement system, which improves the flexibility and adaptability of the system. Based on the digital optical bench system, the automatic generation method of the digital image target is studied. Based on the principle of digital correlation, the use of sine-distributed periodic targets, the high-precision automatic extraction algorithm of digital image targets is studied, and good robustness and repeatability error of 8×10-5 are shown. The method of calculating the period by digital fitting is used to measure the focal length of the small field of view optical system. The experimental results show that the relative error of this method is 0.24%, which can be used for the focal length measurement of the small field of view optical system.