As the warship target always emerges nearby the sea-sky-line,obtaining the location of sea-sky-line can determine the target′s potential area in the complicated sea-sky background.Based on the gray-scale transition character of sea-sky area,we propose a sea-sky-line locating method in this paper.We first adopt the morphological opening operation to suppress high-frequency target and clutter to extract the smooth background,then,by detecting the first anticlimax of the background row-mean curve,we get a narrow region,which is corresponding to the sea-sky area,and take the mid-line of it as the approximate sea-sky-line within engineering tolerance.The experimental results show that our algorithm can successfully locates the sea-sky-line in complicated sea-sky background with high accuracy.