Abstract: A fast clustering algorithm for thresholded image is proposed in the paper.It has five features: ① the image scanning process and clustering process are combined into one process; ② it is fast; ③ the total number of the clusters is created dynamically; ④ the space needed is less; and ⑤ it is convenient for later processes.It can be applied to the field of target detection,multi targets tracking, and noise removing.