When to detect the air target at the low altitude by the infrared (IR) detecting system, the scenery on the ground can also be contained in the view of the detector, and affects detecting of the target. Based on the IR imaging, the problem of the ground scenery can weaken the detecting capability of the IR system is studied, and the reason for this problem is analyzed, the method of graying a special signal range within the view of the detector is proposed for this specific problem. According to this method, the radiation of the target and its background are evaluated, the length and the location of the signal range to be grayed are confirmed by the evaluated results, so the signal range that need to be grayed can be gotten. The calculated and experiment results show that the detecting capability can be improved by this method when encountering the ground scenery.