Aiming at the problem of real-time autonomous obstacle avoidance of underactuated USVs under unknown environment with multiple dynamics obstacles, an online obstacle avoidance path planning algorithm was proposed. Based on the guidance of the global path, the constraints of multiple obstacles, of USV ontology kinematics and dynamics, and of the established air route were taken into full consideration. At the same time, the multi-objective function was taken as the optimization function to perform online real-time obstacle avoidance through obstacle motion prediction and rolling optimization. The correctness of the algorithm was verified by simulation experiments and analysis. A certain USV was used to test the obstacle avoidance algorithm on the lake, and the test results verify the effectiveness of the algorithm in practical engineering applications.