Aimed at the problem of extracting the region of interest of ship target from the infrared image, A fast ROI (Region of Interest) acquisition method is proposed. Firstly, use the median filter, Canny edge detection and Hough transform to detect the sea-sky-line/coastline from infrared image. Secondly, divide the region of sea-sky-line/coastline. Finally, determine the ship's ROI through the area characteristics of ships. The algorithm is simulated and verified by using real infrared image. The results show that the method is simple and intuitive, and it can effectively determine the ROI of the ship target in the near shore.