In this paper, an adaptive tracking controller is proposed for a class of nonlinear systems with external disturbance. Unlike existing approaches, the considered system is transformed into a fully actuated system (FAS) model, simplifying the design of tracking controller. Additionally, this paper presents an extended state observer (ESO) that estimates both external disturbance and system states by defining the disturbance as a generalized system state. The error dynamic system is established by the designed reduced-order ESO. By combining the approach of linear matrix inequality (LMI) and backstepping, a tracking control law is constructed to guarantee the stability of the error dynamic system. Furthermore, we also discuss the specifics of tracking controller design. Theoretical analysis of stability demonstrates that the proposed tracking control scheme can ensure that the tracking error converges to zero, and all closed-loop system signals remain bounded. The simulation results illustrate the effectiveness of the suggested algorithm.