According to the features of sea-sky-line in an infrared image,a sea-sky-line detection method is proposed.First,medium filtering is used to restrain the noise in an infrared image.Secondly, the infrared image coped with medium filtering is processed by using a line detection template so as to detect all of the horizontal lines in the image.Finally,the detected infrared image is scanned line by line to identify the position of the sea-sky-line.The experiment shows that this method can identify the position of the sea-sky-line and decrease the difficulty and complexity in the subsequent detection of maritime targets.