In　order　to　ensure　the　safety　and　reliability　of　the　mechanical　performance　of　the　whole　bridge　in　the　operation　stage　of　the　completed　bridge,　this　paper　takes　an　Extradosed　bridge,　Panzhihua　Jinshajiang　bridge　of　Chengdu-Kunming　Railway,　as　the　research　background,　analyzes　and　studies　the　longitudinal　resonance　dynamic　characteristics　of　the　short　tower　cable-stayed　bridge　structure　when　the　train　load　acts　on　the　main　beam　under　the　condition　of　completed　bridge.　The　longitudinal　vibration　of　the　main　girder　caused　by　live　load　will　directly　affect　the　safety　and　use　of　the　expansion　joint　device　at　the　abutment,　and　the　damage　of　the　expansion　device　will　lead　to　the　safety　and　reliability　of　vehicles　in　the　later　operation　stage.　The　dynamic　response　of　the　structure　caused　by　the　different　speed　of　live　load　is　analyzed.　The　results　show　that　when　there　is　no　significant　difference　between　the　live　load　action　speed　and　the　estimated　longitudinal　bridge　resonance　speed,　there　is　no　significant　difference　between　the　longitudinal　bridge　loading　frequency　when　the　live　load　passes　through　the　bridge　and　the　first-order　longitudinal　vibration　frequency　of　the　bridge.　A　longitudinal　resonance　phenomenon　can　be　observed　in　Jinshajiang　bridge,　in　which　the　dynamic　response　of　the　main　beam　and　the　main　tower　is　obvious.　©　2020　IEEE.