To optimize the trajectory planning problem of underactuated ship automatic berthing and avoid getting stuck in local optima, a real-time trajectory planning method considering the overall constraints of the ship kinematic model and obstacle constraints was proposed. Based on the gridded map, the A* algorithm was used to plan the global optimal path, and the redundant point deletion strategy was applied to obtain the reference waypoints for trajectory planning. Considering that ship berthing was a dynamic process, the extended dynamic window algorithm (EDWA) was used to predict the berthing trajectory of the ship during the constant force stage and deceleration stage, and the proportional integral derivative (PID) control method was introduced to adjust the ship’s heading. After conducting feasibility tests on all predicted trajectories, the optimal trajectory was obtained by using an evaluation function that considers constraints such as target points, reference waypoints, and obstacles, and the corresponding force was used as the control input for the ship. Repeat the above steps for realtime planning based on the current control input and motion state of the ship at the next sampling time until the ship reached the target point. The feasibility of the proposed method was verified by using the simulation scenario of Rizhao port. Results show that the proposed method can guide ships to avoid obstacles and reach the designated berth in a relatively short time, the distance between the trajectory and the nearest obstacle is 144.1428 m, and the trajectory length is 47.26 m longer than the traditional A*+EDWA, however, the planning time is reduced by 55.12%, which show the superiority of the proposed method.