A robust H∞ adaptive control method based on backstepping theory was proposed for position servo system of asymmetric hydraulic cylinder in the presence of parameter uncertainties and external disturbances. In the controller design process, a proper Lyapunov function of the subsystem was selected to avoid the problem caused by repeated differentiations of high-order system. The robust control method guaranteed the output tracking error satisfied the H∞ performance. Considering the uncertain parameter in front of the control input could lead to the nesting problem caused by the mutual inclusion of the control law and parameter adaptive law, the uncertain parameter in the model of the system is replaced and a proper Lyapunov function was constructed, which solved the problem. Finally, the stability of the closed loop system was proved based on Lyapunov function approach. Simulation results verified that the proposed controller enabled the output of the closed loop to track the given signal at a faster speed, to achieve smaller tracking error and better robustness to parameter variety by contrast with PID control method.