More and more aerial imaging devices appear with the development of aviation technology,and imaging devices demand more and more high focusing precision. This paper discusses an autofocus method of aerial imaging device,in which,principle of autofocus control and grating method are analyzed in detail,flow chart of autofocus system in whole aerial imaging device is given out and off-focus quantity allowed is computed and verified in experiments,and indicates how to proceed with automatic focusing compensation at the actual photographic process. The experimental results show that using the method of automatic focusing,focusing precision is less than 0.02 mm,focusing time around 5 s.