Abstract: To solve the problem of auto-focusing in large array airborne remote sensor, a method to search position of focal plane in center field through auto-focusing in margin field was presented. Combination of oblique assembling and fine setting were taken to improve precision of auto-focusing, which could settle the influence of field angle in margin field. Microscope adjustment, photographic adjustment and electric adjustment were used to realize coarse calibration, accurate calibration and fine calibration of focal plane position and the problem of difference between focal plane position in center and in margin field was resolved. In servocontrol of the auto-focusing system, coarse-to-fine and ergodic hunting method were taken to find the optimum position of focal plane. Minimum step length was 0.01mm, and repeatability precision was 0.03mm, which was within focal depth and met the requirement. After experiment and flight test, focal plane that was detected by this method was accurate and the photographs were in high quality.