Aiming at the problem of complies with the Convention on the International Regulations for Preventing Collisions at Sea and online real-time path planning of unmanned surface vehicle in dynamic environment. A new method of online path planning is proposed for Unmanned Surface Vehicle in a dynamic environment. This method have merged International Regulations for Preventing Collisions at Sea (COLREGs) with Velocity Obstacle (VO), make relative speed of velocity obstacle avoidance method lineation, and make VO as a set of velocities constraints combined with mixed-integer linear programming. At mealtime, combined USV body dynamics constraints and environmental constraints, and make multi objective function as the optimization function and choose distance, speed and abeyance COLREGs as target functions. At last, analysis the path planning about two kinds of encounter situations and the simulation results show the effectiveness of this planning algorithm.