Taking into account the strong nonlinearity associated with the dynamics of the electrohydraulic system, a passivity based nonlinear robust control method was developed for it. In the proposed method, the Lyapunov function was designed with the sliding mode of tracking error and the pressure error storage function constructed in the hydraulic passivity theory. Thus the backstepping design process can be decomposed into two cascaded steps: position tracking and pressure tracking. The electrohydraulic system has a redundant dimension, as the two chambers of the hydraulic actuator have different areas. The added internal dynamics of pressure also brought the stability issue. To handle this difficulty, a steady state working point based assignment strategy was presented. This strategy combines force and flow constraint conditions, such that it can assign unique desired pressure for each chamber of the hydraulic actuator. As a result, the nonlinear robust control law was synthesized. Experiments were carried out on a prototype system to validate the effectiveness of the proposed method. The experiment results show that with the developed controller, the electrohydraulic system achieves accurate tracking. The error is no more than 5μm, while tracking constant speed and sinusoidal trajectories. The proposed control method presents good tracking performance and performance robustness. 