In this paper, a serial strategy based on the directional A* algorithm for multiple UAVs is proposed. Firstly, the directional A*algorithm is presented to create a smooth and feasible pre-planning route, in which the maximum pitch and yaw angles are used as search constraints of the A* algorithm to narrow expansion area of nodes, and the successive trial and error procedure is adopted to avoid "dead zones". Then, the serial re-planning algorithm is discussed to extend the path length for aggregating at the same time, which generally consists of three steps: variable step size and multiple nodes, three-dimensional circle maneuver and virtual threats. Finally, the effectiveness of the proposed algorithm is demonstrated using numerical simulation.