The　safety　issue　involving　unplanned　H2　leakage　has　retarded　promotion　of　Hydrogen　fuel　cell　vehicles　(HFCVs).　When　an　accidental　leakage　occurs　in　a　relatively　closed　space,　H2　tends　to　accumulate.　It　is　necessary　to　obtain　a　better　understanding　of　the　H2　dispersion　characteristics　in　a　relatively　closed　space.　Most　studies　focus　on　the　dispersion　features　in　a　small　garage,　whereas　few　consider　large　underground　garages　with　complex　ventilation　systems.　Herein,　H2　released　from　a　pressurized　tank　in　a　real　large　underground　garage　was　investigated　by　a　two-stage　computational　fluid　dynamics　(CFD)　study.　An　isentropic　expansion　model　with　a　real　gas　equation　of　state　was　employed　in　stage　(1)　to　estimate　the　source　strength　which　can　be　used　as　inlet　boundary　conditions　for　stage　(2),　the　dispersion　stage.　The　CFD　models　were　validated　against　the　experimental　results.　The　concentration　field　and　flammability　envelope　were　focused.　This　study　provides　a　feasible　method　for　assessing　the　risks　associated　with　HFCVs.　©　2022　Hydrogen　Energy　Publications　LLC