Infrared small target detection algorithm based on bandpass filtering is presented. The target imaging speciality is analyzed correctly, and the design method of bandpass filter is discussed. The target is confirmed by the correlation method based on position and gray after filtering process, and the calculation model of sea-skyline is introduced. This method can properly restrain the interfere of sea and clouds, reduce the cost of computation, track the target in real-time, and can be implemented by hardware easily. It is very useful for the detection of small moving targets with low signal noise ratio.