Venting　of　hydrogen-air　deflagrations　is　a　complex　process,　and　many　issues　remain　to　be　investigated.　In　order　to　analyze　the　mechanism　and　influence　factors　of　the　overpressure　development　during　vented　hydrogen　deflagrations,　a　commercial　code　FLACS　was　used　and　the　capability　of　the　code　was　validated　by　previous　experimental　data.　Based　on　the　experimental　and　numerical　results,　the　effect　of　concentration,　ignition　location　and　vent　area　on　the　vented　overpressure　was　discussed　in　detail.　It　was　confirmed　that　in　the　condition　of　the　large　vent　area,　three　overpressure　peaks　are　formed　at　the　moments　of　the　vent　failure,　the　external　explosion　and　the　occurrence　of　the　maximum　flame　surface　area　in　the　vessel,　which　are　marked　as　Pburst,　Pext　and　Pmfa.　The　overpressure　peak　Pext　is　corresponded　to　the　formation　of　the　external　pressure　caused　by　the　external　explosion,　and　peaks　Pext　in　BWI　conditions　are　larger　than　those　in　CI　conditions.　The　relationship　between　overpressure　and　vent　area　match　the　power　law　with　negative　exponent,　while　the　larger　vent　area　may　lead　to　the　stronger　effect　of　the　external　explosion　on　the　internal　overpressure.　Moreover,　the　differences　in　magnitude　between　Pext　and　Pmfa　were　discussed.　©　2019