In order to solve these problems that the traditional method of frame difference method often appears 'void' and false object, this paper proposed a novel method of object detection which combined 7 frame symmetric difference and Top-Hat transformation, analyzed the fundamentals of the general frame difference, 7 frame symmetric difference, combination of 7 frame symmetric difference and Top-Hat, and offered the algorithms implementing procedures. The whole experimental results of analysis are based on Matlab 2010b. The experimental results show that the method has a good real-time performance and a stable performance in object detection, and it can be applied to infrared object detection in condition of ground to air.