Abstract: In order to detect faults of electro-hydraulic servo system effectively, it is important to design an ascendant observer beforehand. This paper presents a method of linear robust observer design for electro-hydraulic valve-controlled single rod cylinder servo system. Firstly, the nonlinear model is applied in this study. Secondly, the linear modeling process is recapped by highlighting the likely uncertainties which influence the implementation of model-based approaches. Finally, the linear robust observer is designed in the presence of unknown inputs, such as model errors, parameters fluctuation, force disturbance, process noise, measurement noise. Case simulation under light running reveals that this observer is being of some robustness.