The imaging quality of the optical system in an aerial camera is significantly affected by the accuracy of focal plane detection. To resolve the focusing problem of aerial cameras, a focusing algorithm based on multiple differential filtering effects is proposed. First, the principle of focusing is expounded. With the basic rectangular filter as the design basis, a new filter suitable for focusing of aerial cameras is proposed to eliminate the influence of the relative displacement between the image and the filter, and its feasibility is proved from a mathematical point of view. Then, the filter is simulated by interlaced sampling of the image to complete the design of the image focusing algorithm. Finally, the images collected in the experiment are used to evaluate the focusing effect of this algorithm. The experimental results show that the proposed algorithm can eliminate the sinusoidal change caused by the relative displacement between the rectangular filter and the image. The proposed algorithm is able to meet engineering requirements with high sensitivity and a maximum error of 41.16 μm, which is smaller than the half focal depth (76.8 μm) of the optical system.