An extended state observer based on sliding-mode control method is proposed to solve the stability problem of quadrotor UAVs with disturbances and uncertainties.First, according to the transformation of body coordinate system and ground coordinate system, the dynamic model of UAV is constructed by using backstepping control method.Then, an extended state observer is designed to restore the state of the system and estimate all disturbances and uncertainties of the system.It can achieve rapid error convergence and high enough estimation accuracy.At the same time, the sliding-mode controller is designed.Combined with Lyapunov stability theory and linear matrix inequality method, the stability condition of the observer is proposed.Finally, the validity is verified by numerical simulation.