Based on the linear extended state observer(LESO), a high-order LESO of high-gain form for the second-order plant is proposed. The convergence of the state estimation error of HLESO and the stability of high-order lineractive disturbance rejection control(HLADRC) is proved. Simultaneously, robustness for input gain uncertainty and model uncertainty are analyzed based on the two-degree-of-freedom(2dof) closed-loop transfer function and frequency response. Then, the region of parameter ?? where the closed-loop system is stable and the relationship between the dynamic characteristics of rejection for external disturbance and controller bandwidth is discussed. Finally, simulation experiment is carried out by comparing with liner liner active disturbance rejection control. The results show that the HLADRC has a stronger anti-disturbance ability and convergence performance, but in this case the LADRC has a better robust stability and performance, establishing both the conceptual and pactical foundation for engineering design.