The　semi-fixed　dowel　joint　studied　in　this　paper　consists　of　a　steel　dowel　with　rubber　sleeves　and　a　rubber　pad　to　connect　the　superstructure　and　the　substructure　and　to　support　the　main　beam.　This　type　of　joint　is　suitably　used　in　retrofitting　a　jointed　bridge　with　high-stiffness　substructure　into　a　semi-integral　bridge.　This　study　aimed　to　investigate　the　nonlinear　stiffness　of　the　joint,　which　is　the　key　parameter　in　the　retrofitting　design.　Taking　the　real　joint　in　a　retrofitted　semi-integral　bridge　as　a　prototype,　tests　on　full-scale　models　were　carried　out,　in　which　the　wall　thickness　of　the　rubber　sleeve　and　the　thickness　of　the　rubber　pad　were　taken　as　parameters.　The　test　results　show　that　the　wall　thickness　of　the　rubber　sleeve　is　the　main　factor　affecting　the　radial　stiffness　and　the　flexural　stiffness　of　the　semi-fixed　dowel　joint.　With　the　decrease　of　the　wall　thickness　of　the　rubber　sleeve,　the　radial　stiffness　decreases　while　the　flexural　stiffness　increases.　However,　the　thickness　of　the　rubber　pad　has　little　influence　on　both　the　radial　and　flexural　stiffness　of　the　joint,　and　its　main　role　is　only　vertical　loading-bearing.　If　the　longitudinal　deformation　of　the　main　beam　of　a　bridge　under　maximum　temperature　variation　is　10　mm　as　the　allowance　value　of　engineering　design,　the　wall　thickness　of　the　rubber　sleeve　for　this　joint　should　not　be　less　than　40　mm.　At　last,　on　the　basis　of　experimental　research　and　theoretical　analysis,　the　formulas　of　radial　stiffness　and　flexural　stiffness　are　fitted;　it　can　be　used　as　reference　for　practical　engineering　application,　or　for　formulation　of　relevant　specifications.