Abstract: Aiming at the problem of aerial camera focal plane detection, an aerial camera focal plane detection method based on out-of-phase difference filter was proposed. Firstly, the principle of automatic focus detection by spatial filtering effect was introduced. Secondly, the out-of-phase differential detection algorithm was used to design out-of-phase differential filters in order to eliminate the fundamental frequency components in the spatially filtered signal, increased the signal-to-noise ratio and amplitude, and improved the focus accuracy. The frequency spectrum of the out-of-phase differential filter’s output signal was analyzed. Finally, guide rail speed between 5-53.2 mm/s according to the typical speed-to-height ratio was set to perform 25 image plane inspections, and the focusing effects of traditional image focusing operators was compared with differential filtering methods. The results show that the differential filter focusing method has strong scene adaptability and high sensitivity, and the maximum error of the focusing system is 45.18 μm, which is less than the allowable error of the optical system of 76.8 μm and meets the engineering requirements.