For compensating the defocus caused by the changes of temperature, pressure and other environmental parameters of an aerial camera in the air, an autofocusing system for aerial camera based on opto-eletronic auto-collimation method was designed. Utilizing raster with high contrast in the camera, this system avoided the situation that autofocusing based on image processing technology failed when object is with low contrast. Modulated by raster in object space, image space and the swing of reflector, the signal of light intensity was easy to be processed. Combined coarseness with fine searching strategy, the system guaranteed the validity of adjustment result. Experimental results show that the autofocusing process is accomplished in less than 20 s, and the repeatability is less than half focus depth, which satisfies the focusing requirement of aerial camera.