A three-degree-of-freedom helicopter attitude tracking control system can be described as a multi-input multi-output strict-feedback form system with unknown parameters, bounded disturbances, and nonlinear uncertain cross-couplings. Both normbounded and nonlinear uncertainties are discussed. The whole nonlinear system is divided into a nominal disturbance-free system and an equivalent disturbance part. A robust control method based on signal compensation technique and backstepping decentralized control is proposed. Robust practical tracking property of closed-loop system is proven, and the tracking error can be made as small as desired with expected convergence rate. Experimental results demonstrate the improved tracking performance of the attitude tracking control system with different types of reference signals, such as step signals, ramp signals, and nonstationary sinusoidal signals.