The　electro-hydraulic　servo　steering　system　(EHSSS)　is　the　key　element　to　determine　the　handling　and　stability　of　heavy　vehicles.　Accurate　steering　control　is　challenged　due　to　its　complicated　structure　and　a　wide　range　of　steering　loads.　In　this　paper,　an　output　feedback　integral　sliding　mode　control　(ISMC)　method　based　on　high-gain　observer　is　proposed　to　solve　this　issue.　First,　a　new　Hurwitz　matrix　design　method　is　presented　for　EHSSS　with　non-chain　integrator,　in　order　to　prove　the　convergence　of　the　observer　error　dynamics.　Then,　the　Lyapunov　stability　of　the　observer-based　controller　is　verified.　Finally,　a　test　bench　is　established　to　experimentally　validate　the　proposed　method?s　effectiveness　through　multiple　test　scenarios.　The　results　show　the　tracking　error　of　the　proposed　method　is　significantly　smaller　than　PI　and　similar　to　full-state　feedback　integral　sliding　mode　controller.　This　paper　provides　a　valuable　reference　for　high-gain　observer-based　nonlinear　control　applied　to　a　typical　electro-hydraulic　servo　steering　system.