For global path planning of UAVs and the evasion of local dynamic obstacles during flight，a real-time dynamic obstacle avoidance algorithm that integrates the improved A^* algorithm with the velocity obstacle method is designed.This algorithm introduces an adaptive node expansion step size strategy based on Euclidean Signed Distance Field(ESDF)map into the traditional A^* algorithm to reduce redundant nodes in the path searching process.The initial path is smoothed by the B-spline curve.A velocity obstacle method in three-dimensional space is designed based on UAV dynamics model，achieving real-time obstacle avoidance in local dynamic environment.An attraction function is introduced between the local obstacle avoidance path and the global waypoints to guide the UAV to smoothly return to the global path after avoiding local obstacles，preventing it from falling into a locally optimal state.The simulation experiments show that the proposed algorithm can save the search time by 64.5% and reduce the number of exploration nodes by 62.2% for global path planning，and the UAV can effectively avoiding obstacles in dynamic environment.