A new method is presented to detect small target in the port-sea background image.The image contains land and water area,and the target is in the water area, so the algorithm of automatic segmenting the area that we are interested in is presented.Firstly the land and water area are divided,then the black-white image which has been segmented is marked and the biggest block is distilled as the water area.Secondly the area＇s contour is scouted and it is filled with the original pixels. A local median median filter is executed to filter the rest bundance background elements.Eventually the real small target can be segmented by the F-R rule.And the experimental result shows that the method is effective