A fast online nonlinear model predictive control (NMPC) approach was applied to trajectory tracking of miniature coaxial unmanned helicopter(MCUH). The nonlinear model of MCUH was simplified. And then, utilizing the differential flatness of MCUH, a linear time varying (LTV) model was employed to approximate the nonlinear dynamic behavior of MCUH, and further on applied in the conversion from classical NMPC schemes to LTV-MPC schemes for output trajectory tracking, resulting in reduction of the complexity of the problem solving. Afterwards, the terminal region and the end penalty were designed to ensure the exponentially asymptotically stable of the trajectory tracking error system. The numerical simulation verified the proposed NMPC control scheme to the time-varying trajectory tracking of MCUH with constraints.