For small targets detection in single frame infrared image, a background clutter suppression algorithm based on an adaptive smooth filter and spatial filter was used to analyse the IR scene model. The method has good performance to detect the small targets in the undulant background and has the merit with no target information loss. At the same time, the algorithm is realized by the digital processor ADSP-TS201S with high performance and successfully applied in an all directional IRST system. The experiment shows that this method has high detection performance for the small target in single frame at low SNR.