Temperature　variation　can　trigger　longitudinal　expansion　and　contraction　deformation　of　the　main　girder　of　concrete　bridges.　To　accurately　predict　the　deformation,　methods　to　calculate　the　average　cross-section　temperature　of　the　main　girder　of　bridges　of　such　type　were　studied.　A　finite　element　model　was　established　by　MIDAS,　taking　a　concrete　small　box　girder　bridge　as　the　prototype　bridge.　The　temperature　field　of　the　bridge　was　simulated　and　the　peak　average　cross-section　temperatures　of　the　small　box　girder　under　extreme　climate　conditions　were　calculated.　Two　simplified　methods　to　calculate　the　average　temperature　of　the　girder　are　proposed,　one　taking　into　account　the　peak　ambient　temperatures　and　the　maximum　daily　solar　radiation　(Method　1)　and　the　other　considering　the　peak　average　daily　temperatures　on　a　monthly　basis　and　the　maximum　daily　solar　radiation　(Method　2).　The　results　indicate　that　the　finite　element　model　built　can　accurately　simulate　the　actual　temperature　field　of　the　structure.　It　is　suggested　that　the　peak　average　cross-section　temperature　under　extreme　climate　conditions　be　used　to　predict　the　main　girder　longitudinal　expansion　and　contraction　deformation　of　concrete　bridge　triggered　by　temperature　variation.　The　peak　average　cross-section　temperature　of　small　box　girder　is　in　positive　linear　relation　with　the　peak　ambient　temperature.　Method　1　can　generate　accurate　prediction,　but　a　little　complex.　For　the　ease　of　calculation,　Method　2　is　a　better　choice,　but　the　accuracy　is　not　as　good　as　Method　1.　©　2021,　Journal　Press,　China　Railway　Bridge　Science　Research　Institute,　Ltd.　All　right　reserved.