A cascade control system is designed to solve the problems of nonlinear

strong coupling

underactuation and sensitive to disturbance in flight control system of four-rotor unmanned aerial vehicle（UAV）.In the control system

the outer position loopadopts the linear active disturbances rejection controller（ADRC）

and the inner attitude loop adopts the improved linear ADRC.In position and attitude control

the designed controller is compared with the linear ADRC and the PID controller by simulation analysis.The simulation results show that under certain external disturbances

the designed controller has a good decoupling effect

which can estimate and compensate the total disturbance well

and improve the anti-interference ability and robustness of the system.