Abstract: An optimal planned route is a foundation of safe and economical navigation. This paper proposes a method for the shortest time routing based on an instantaneous depth model. Firstly, an instantaneous model is constructed based static depths in a chart and a tidal model in a sea area, then obstacles are acquired and the navigability of a route is analyzed. Secondly, the shortest time path is searched using a dynamic programming method. Finally, the path is optimized by the strategies including excluding obstacles outside the rectangle border of all nodes, searching close nodes in next stage and deleting redundant nodes for a more direct navigation. Experimental results demonstrate: (1) the proposed method can analyze the navigability of the routes dynamically and acquire the shortest time route; (2) the route acquired by the proposed method has the shortest time than that by the traditional methods; (3) the shortest route depends on the start time of navigation and the ship speed.