A cooperative trajectory planning algorithm of Multiple unmanned aerial vehicles (UAVs) based on Multi-Agent Proximal Policy Optimization (MAPPO) has been proposed for the three-dimensional cooperative trajectory planning. Firstly, considering the constraints of cooperative trajectory planning and the battlefield environment, a three-dimensional cooperative trajectory planning model is established. Secondly, based on the established trajectory planning model, the state space, action space, and reward function are designed to implement cooperative trajectory planning. Finally, simulation is carried out to verify that the planned trajectory satisfies the environmental and spatial cooperative constraints and is flyable.