Abstract: As for the trajectory tracking problem of unmanned helicopters impacted by the disturbances, a composite nonlinear model predictive control scheme based on a harmonic observer is proposed. Firstly, the trajectory tracking problem is transformed into the tracking problems of position and attitude commands. And then, a harmonic disturbance observer is designed to estimate the periodic disturbances. The composite nonlinear model predictive controller is designed based on the nonlinear model predictive control algorithm combining estimation information. Finally, the proposed scheme is tested on a practical small-scaled helicopter under the simulation environment of MATLAB. The simulation results indicate that the proposed scheme not only achieves effective suppression of periodic disturbances, but also guarantees the high-precision trajectory tracking of unmanned helicopters.