In this study, a non-linear backstepping control method based on a high-gain observer design is presented for a single-rod electro-hydraulic actuator, which is not a strict feedback non-linear system. An equivalent model transformation is used for load pressure to establish the relationship between displacement command and virtual command of load pressure. So the convergence analysis of the single-rod electro-hydraulic servo (EHS) model can be divided into two parts. Since the EHS has only one measured output state, a high-gain observer is developed to estimate the full-state of EHS, which can be used in the design of backstepping control. Furthermore, the convergence of this proposed controller is proved by Lyapunov technique. The experimental results verify the dynamic behaviour of this controller varying the observer parameters. In comparison to proportional-integral controller, the proposed controller is more suitable for this EHS in some critical conditions with high response frequency and large unknown external load.