Abstract: The equations of ship domain boundary with 4 arguments are built, involving ship speed, turning performance, anti-collision action and critical passing distance. The ship domain structure model is constructed according to numerically defined collision urgency distribution. The shape of the model is different to ordinary ship domain. It is bigger at bow area and starboard side and smaller at stern area and port side and the sizes depend on maneuverability performance of a ship and the target approaching speed. The better of the maneuverability performance, the smaller the sizes, and the higher the approaching speed, the larger the sizes. The represented collision urgency within the domain goes lower gradually outwards. The lowest rate of degrease is seen at the bow area and the rate becomes higher in order of bow-starboard-stern.