Detection of dim target in infrared image is the emphasis and difficulty of target detection. A method of detection using the Top－hat operator of morphology is proposed. Its main procedures are filtering the infrared image sequences, and then detecting the targets depending on the continuity of the movement of the targets. Some features of detected target are analyzed. The effect of the detection method is verified by the results of experiments. The characteristics of the gray and centroid of small target in infrared image sequences are also found, which is significant to improve the precision of detection.