This paper introduces a novel path planning algorithm for Unmanned Aerial Vehicle (UAV) based on theory of fluid avoiding obstacles.To the problem of path planning which aims at the global optimum,not only the influence of terrain constraints on path safety but also the performance constraints of UAV should be taken into consideration,thus a feasible and smooth path will be planned in the three-dimensional space.The computation complexity and the path quality by traditional algorithms are usually unsatisfactory,hence we propose the method inspired by the phenomenon that water can avoid rocks and reach the destination.The common features between this phenomenon and the path planning problem are extracted and analyzed,and then the mathematical model generalizing the phenomenon is constructed on the basis of theory of fluid mechanics.By selecting the optimal streamline from the fluid field under the evaluation index,the three-dimensional optimal path under flight and environment constraints is obtained eventually.