The　transportation　and　utilization　of　hydrogen　blended　natural　gas　have　received　extensive　attention.　The　dangerous　characteristics　of　hydrogen　such　as　high　diffusivity　and　wide　flammability/explosion　limit　also　increase　the　leakage　risk　of　hydrogen　blended　natural　gas.　In　this　paper,　a　numerical　model　is　established　for　the　leakage　and　diffusion　of　hydrogen　blended　natural　gas　in　a　closed　container.　The　evolution　of　the　distribution,　diffusion　law　and　flammable　area　of　different　proportions　of　hydrogen　blended　natural　gas　after　leaking　into　a　closed　container　is　investigated.　The　results　show　that　the　flammable　area　with　low　hydrogen　ratios　(20%　and　below)　will　disappear　within　2.7　s–11.1　s　after　the　leakage,　which　is　relatively　safer,　while　the　high　hydrogen　ratio　(80%　and　above)　reaches　3875　s–4555　s　with　a　significant　increase　in　risk　duration.　After　the　50%　hydrogen　ratio　leakage,　the　thickness　of　the　flammable　area　is　higher　than　15.67%　for　the　80%　hydrogen　ratio　and　30.25%　higher　than　pure　hydrogen　at　120　s　after　leakage,　and　the　risk　is　higher　in　a　short　time.　Due　to　the　difference　in　the　diffusion　rates　between　methane　and　hydrogen,　hydrogen　diffuses　to　the　middle　and　lower　part　of　the　enclosed　container　faster,　and　the　risk　in　the　middle　and　lower　part　also　deserves　attention.　©　2022　Hydrogen　Energy　Publications　LLC