To solve the problem that model bias and external disturbances increase the difficulty of position tracking and anti-swing control of the slung-load flight system of quadrotor UAVs,a nonlinear control method based on compensation function observer is proposed.Firstly,the dynamic model of the slung-load flight system of a quadrotor UAV is built and a compensation function observer is designed to observe the unmodelled dynamics and unknown disturbances.Then,in view of the tracking error constraints of system reference inputs,a barrier Lyapunov function is built to design a backstepping controller,and the total disturbance is compensated for by introducing the estimation value of the compensation function observer in the controller,thus ensuring the UAV position errors and the slung-load swing angle to change within the constraint ranges and making the system achieve the expected control effect.Lyapunov stability analysis proves the stability of the closed-loop system,the convergence of position errors and the suppression of slung-load swing.The simulation results show that the proposed controller can effectively suppress the swing of the slung-load while realizing the accurate control of UAV positions.