An electro-hydraulic steering system of agricultural vehicle was designed for automatic navigation based on a Maoyuan-250 tractor. An electro-hydraulic proportional valve, a directional valve and a relief valve were used to reconstruct the oil circuit of the original steering system of the tractor. And the electro-hydraulic proportional valve was controlled by a STC12C5A60S2 while an angle sensor was applied for feedback. A steering model based on PD control algorithm with dead zone was established by SimHydraulics, and experiments about left-and-right limit and four specific target angles were conducted on the real vehicle. The tests showed that the response time was reduced by 0.2 s and the overshoot was controlled within 5% compared with the original system.