The reference model tracking control of an unmanned aerial helicopter（UAH） under outside disturbance is studied， and a tracking control scheme based on the adaptive disturbance observer is proposed. Firstly， the disturbance is modelled as a nonlinear exogenous system with uncertain parameter error based on the available information. Secondly， a state observer and an adaptive disturbance observer （DO） are proposed to estimate the unmeasurable state and disturbance of the UAH system. Thirdly， by combining the reference model control and DO-based control， a composite anti-disturbance control scheme is presented to derive an overall closed-loop system consisting of tracking error and observer error. Fourthly， based on the Lyapunov stability theory， a sufficient condition on H_∞ performance index and asymptotic stability is obtained， and a co-design method of the observers， controller， and adaptive law is proposed by resorting to matrix transformation. Finally， numerical simulations demonstrate the effectiveness and superiority of the proposed tracking control algorithm.