Abstract: Starting from the pixel of highest gray level and computing the disparity in grey level of each two adjacent pixels within the same column of image,the gray levels of pixels where the disparity reaches maximum and minimum are taken as the upper and lower thresholds of image edges in that column.Thresholds are searched in each column of image.Computation complexity of new algorithm is analyzed.Experimental results show that the new algorithm extracts the edge of thermal image quickly and meets the practical demands of automatic target recognition.