Ackermann　steering　is　important　for　the　steering　performance　of　heavy　multi-axle　vehicle.　When　Ackermann　steering　condition　is　not　satisfied,　it　will　lead　to　abnormal　tire　wear.　Traditional　trapezoidal　mechanism　of　heavy　multi-axle　vehicle　is　a　single　degree　of　freedom　(DOF)　mechanism,　which　is　difficult　to　completely　realize　Ackermann　steering.　In　this　paper,　a　new　two　DOF　electro-hydraulic　servo　steering　system　(TDEHSSS)　by　using　a　variable　length　tie　rod　is　proposed　for　solving　the　issue.　First,　a　complex　nonlinear　dynamic　model　of　TDEHSSS　is　established.　This　model　includes　the　two　DOF　mechanical　model　based　on　a　Lagrange　equation,　the　valve-controlled　double　steering　power　cylinders　model　and　the　valve-controlled　tie　rod　cylinder　model.　Then,　a　simulation　model　is　built　through　MATLAB/Simulink　and　the　simulation　results　show　that　TDEHSSS　can　realize　the　proposed　requirement.　At　last,　a　test　bench　is　founded　to　verify　model.　It　is　indicated　that　the　simulation　and　experimental　curves　are　consistent,　showing　that　mathematical　model　is　in　accordance　with　the　experimental　system.　This　research　is　valuable　for　analyzing　TDEHSSS,　designing　advanced　controllers,　and　finally　realizing　Ackermann　steering　for　heavy　multi-axle　vehicles.