Sea-sky-line detection can be implemented by effectively suppressing the background and enhancing the target in an image according to the features of sea-sky environment. An improved method for wiping off crass points is proposed on the basis of the linear fitting algorithm. The method is simulated with the Matlab software. The simulation result shows that this improved method can effectively detect the positions of the sea-sky-line and ships against the sea-sky background. It provides a good basis for target recognition and image registration.