Unmanned Aerial Vehicle (UAV) formation control represents a form of collaborative crowd behavior. This paper proposes a distributed event-triggered tracking control method for multi-UAV formation operating in complex environments. To address the challenges of an unknown leader motion state, bounded external disturbances, and obstacle avoidance, a composite control architecture is constructed that synchronously integrates state estimation, cooperative control, and communication optimization. Firstly, an Extended State Observer (ESO) is designed to estimate the leader’s unknown time-varying velocity and control input, while a Sliding Mode Disturbance Observer (SMDO) achieves precise compensation for unknown external disturbances. Secondly, a distributed cooperative controller without predefined geometric configurations is proposed. This controller integrates tracking error feedback, neighbor velocity coordination, and potential field functions to achieve self-organized formation generation and unified handling of collision/obstacle avoidance. Subsequently, a state-error-threshold-based event-triggered mechanism is established, significantly reducing communication load. Lyapunov theory proves the global asymptotic stability of the system and excludes Zeno behavior. Simulations demonstrate that the proposed control strategy enables the UAV formation to cooperatively track a maneuvering target both with and without obstacles, while effectively reducing inter-agent communication frequency. Future work will extend the method to scenarios under cyber-attacks and further validate it through physical UAV experiments.