The event-triggered tracking control for large-scale high order nonlinear uncertain systems, whose state information is immeasurable, is investigated via an observer-based approach. Firstly, a neural observer is designed to estimate the unmeasurable state information of high order nonlinear systems. Then, a relative threshold event-triggered strategy is proposed to reduce the communication burden between the actuator and the controller. On this basis, a novel observer-based adaptive event-triggered controller is designed to achieve the output tracking of the reference trajectory via the backstepping technique. Theoretical proof shows that the proposed controller guarantees the stability of the closed-loop systems and the Zeno-behavior can be excluded. Finally, some simulation examples are performed to illustrate the effectiveness of the proposed method.