The problem of detecting small target in infrared video sequence was addressed.Instead of being regarded as prominence or high frequency part,the inherent isolated attribute of small target was taken into account and redefined as small isolated regions in image.Based on this idea,a new measurement called Small Target Isolation Degree(STID) was developed for evaluating the likelihood of a pixel as the center of a small target.By calculating STID in every pixel in image,the background clutter was suppressed.Then a novel adaptive threshold was proposed to extract the precise location of small target.Incorporated with the two methods,a single frame small target detection algorithm was built.Its high performance was then proved in a serial of experiments.