Mission being changeable in the process of flight for UAVs formations, a trajectory online planning method is proposed for unmanned aerial vehicle formation based on improved consensus theory. Firstly, the consensus algorithm is improved，and by introducing the artificial potential field method, a collision avoidance strategy is proposed in consideration of inter-vehicle distance and relative velocity. Additionally, a turning strategy is proposed in consideration of the UAV’s relative position, state, turning radius, and speed . Furthermore, an online trajectory planning method is presented based on the concept of formation disassembly and the RRT* algorithm. The simulation results show that the collision avoidance strategy can make the intervals of UAVs a safe distance between UAVs before formation is made, preventing collisions, and the turning strategy makes the formation integral in the process of turning, and the trajectory planning method online for formation recombination can handle the execution of new tasks.