Abstract: Aiming at the features of infrared image obtained by imaging anti-ship missiles, this paper researches on the infrared radiation characteristic of the ship target and background of sea and sky, and makes a conclusion that the infrared image of ship target has an important characteristic, sea-horizon, which can locate the target region. To locate the ship target, the paper studied the sea-horizon feature of ship-target infrared image, and an algorithm based on preprocessing and linear fitting to locate the horizontal feature was put forward. The image preprocessing was carried out by applying the edge detection to obtain the gradient image, then the linearization and thinning processing was operated. At last, the Hough algorithm was used to locate the sea-horizon. The experiment result shows that compared with the improved linear fitting method, this method can extract the sea-horizon characteristic in ship-target infrared image precisely under the condition of low contrast and disturbance of cloud.