The yaw dynamics of helicopter involves input nonlinearity, time-varying parameters and the couplings between main and tail rotor. With respect to such a complicated dynamics, the normal control is difficult to realize good tracking performance while maintaining stability and robustness simultaneously. In this paper, an active disturbance rejection control (ADRC) method is proposed. We design an extended state observer (ESO) to estimate the impact from parametric uncertainties and disturbances, and to realize online compensations. Time-varying parameters and other uncertainties in the helicopter dynamics can be rejected by active disturbance rejection control to achieve small tracking error. The simulations results with respect to the dynamics identified from a real helicopter-on-arm testbed are presented. The simulation results demonstrate the effectiveness of the proposed algorithm under the time-varying arameters and other uncertainties.