Urban　composite　bridges　often　experience　fatigue　failure　risk　induced　by　congested　vehicle　idle　vibration.　To　address　this　problem,　ABAQUS　was　adopted　to　establish　a　vehicle-bridge　coupled　finite　element　model　under　congested　vehicle　loading,　along　with　dynamic　response　analysis　and　stress　cycle　counting.　Regarding　the　total　strain　energy　density　as　a　damage　parameter,　the　cumulative　fatigue　damage　of　structural　details　in　the　steel　box　girder　of　an　urban　composite　bridge　under　four　congestion　load　cases　was　calculated　relative　to　the　damage　caused　by　a　standard　fatigue　vehicle　in　an　unobstructed　state.　The　influence　of　vehicle　congestion　on　the　fatigue　performance　of　an　urban　composite　box　girder　bridge　was　comprehensively　evaluated.　The　results　demonstrate　that　elastic　strain　energy　substantially　contributes　to　the　total　strain　energy　density　of　urban　composite　girder　bridges　under　vehicle　congestion　loading;　hence,　the　fatigue　behavior　is　typical　high　cycle　fatigue,　whereas　fatigue　damage　is　mainly　cumulated　within　3-5　high　stress　range　cycles　at　the　onset　of　idle　vibration.　Accumulated　fatigue　damage　to　the　structural　detail　at　the　bottom　flange-to-web　weldment　is　2-5　times　that　to　the　base　metal,　the　fatigue　performance　of　the　weldment　is　therefore　more　vulnerable.　When　typical　congested　traffic　flow　contains　only　cars　and　buses,　fatigue　damage　can　be　safely　evaluated　by　the　idle　vibration　of　a　standard　fatigue　vehicle;　however,　in　some　unrestricted　road　sections,　fatigue　damage　increases　significantly　due　to　truck　congestion.　Since　the　dynamic　effect　of　engine　idle　vibration　increases　the　equivalent　stress　amplitude　by　65%,　fatigue　damage　caused　by　a　standard　fatigue　vehicle　under　single　congestion　is　14.7　times　that　caused　by　single　crossing　under　unobstructed　conditions,　thus　increasing　the　risk　of　fatigue　damage　of　structural　details.　Cumulative　fatigue　damage　increases　with　the　influence　of　vehicle　congestion,　which　is　affected　by　congestion　load　case,　traffic　volume,　and　congestion　duration.　When　daily　congestion　lasts　for　4　h,　fatigue　damage　owing　to　congestion　can　reach　3.5　times　that　without　congestion.　This　study　can　provide　a　reference　for　the　fatigue　assessment　of　similar　bridges　and　basic　theoretical　support　for　the　safe　operation　of　existing　urban　steel　bridges.　©　2021,　Editorial　Department　of　China　Journal　of　Highway　and　Transport.　All　right　reserved.