针对受外部干扰和模型不确定性影响的四旋翼飞行器的姿态控制问题，提出一种基于固定时间的降阶扩张状态观测器（reduced-order extended state observer, RESO）和固定时间反步状态反馈控制律的自抗扰姿态控制方法。基于四旋翼动力学模型及自抗扰（active disturbance rejection control, ADRC）理论，首先利用系统输出可测特性，设计具有固定时间收敛性能的降阶扩张状态观测器并对观测器收敛性进行理论证明；其次，在反步控制的基础上结合固定时间理论，设计固定时间反步状态反馈控制器，并理论证明了闭环系统的跟踪误差能在固定时间内收敛至原点。最后，通过仿真算例，从收敛时间、稳态误差、最大误差以及均方误差等指标考察了所提出的控制方法的性能。仿真结果表明：所提出的改进方法相较于传统自抗扰控制及反步滑模控制具有更好的控制品质。

For the attitude control problem of quadrotor affected by external disturbance and modeling uncertainty, an active disturbance rejection attitude control method which consisted of a fixed-time reduced-order extended state observer (RESO) and a fixed-time backstepping state feedback controller was proposed. Based on the quadrotor dynamics model and active disturbance rejection control(ADRC) theory, firstly, a RESO with fixed-time convergence property was designed and the convergence of the observer was proved theoretically. Secondly, a based on the backstepping control theory, a fixed-time backstepping controller was designed combined with the fixed-time control theory, and it was theoretically proved that the tracking error of closed loop system can converge to the origin at fixed time. Finally, the performance of the proposed control algorithm was investigated from the indexes of rise time, steady-state error, maximum error and mean square error through simulation cases. The simulation results demonstrate that the proposed improved method has better control quality compared to the traditional ADRC and backstepping sliding mode control.