Aiming at solving the dynamic collision avoidance problem for unmanned surface vehicle under complex marine status， this study constructs the obstacle model and the collision risk model to achieve the track re-planning of dynamic collision avoidance in complex sea conditions with the improved velocity obstacle method that introduced multiple constraints.Firstly， an obstacle model description method for rectangular bounding box was proposed for USV's geometric characteristics of large aspect ratio.A relative motion model of USV in hull coordinate system was established with the relative motion parameters calculated. Secondly，the collision risk was calculated to determine the time to start avoiding the obstacle.Lastly， improved obstacle avoidance path was constructed by introducing obstacle model constraints and characteristic constraints of USV’s autonomous local obstacle avoidance system based on velocity obstacle method. The track re-planning results were obtained through experiments.The simulated experiment on the physical simulation platform of V-REP （Virtual Robot Experimentation Platform） and the actual ship obstacle avoidance experiment show that the path re-planning time during collision avoidance is less than 15 s at a speed of 12 knots， meeting the dynamic collision avoidance requirements of unmanned surface vehicle under multiple constraints of actual navigation.