The　traditional　efficiency　analysis　method　of　hydraulic　system　generally　neglects　the　influence　of　mechanical　nonlinearity,　which　causes　great　difference　between　theoretical　efficiency　optimization　and　actual　application.　This　will　reduce　the　energy-saving　effect.　In　this　paper,　the　electro-hydraulic　servo　steering　system　(EHSSS)　of　heavy　vehicles　is　taken　as　the　research　object.　Considering　both　mechanical　nonlinearity　and　hydraulic　nonlinearity　of　the　steering　system,　this　paper　establishes　the　efficiency　model　of　the　EHSSS.　In　order　to　achieve　the　efficiency　analysis　of　this　nonlinear　model,　an　efficiency　analysis　method　in　quasi-steady　state　condition　is　proposed.　The　theoretical　analysis　is　consistent　with　the　experimental　results　in　four　different　single-side　loading　conditions,　so　the　validity　of　the　model　and　method　is　verified.　Then,　based　on　the　above　model　and　method,　the　system　efficiency　spectrums　are　obtained　in　different　operating　conditions.　The　efficiency　of　the　EHSSS　with　strong　nonlinearity　is　not　only　related　to　the　equivalent　load　but　also　the　steering　angle.　The　results　show　that　the　efficiency　is　the　highest　in　the　maximum　equivalent　load　conditions　and　at　left　or　right　end　points.　In　addition,　this　paper　obtains　the　efficiency　curves　of　steering　system　in　different　speeds,　hydraulic　parameters　and　structural　parameters.　The　method　presented　in　this　paper　takes　into　account　the　effect　of　mechanical　nonlinearity　on　the　efficiency,　which　provides　theoretical　guidance　for　further　efficiency　improvement　of　electro-hydraulic　steering　system.