Although　ammonia-hydrogen　blended　fuel　has　become　a　research　hotspot,　it　is　essential　to　recognize　that　adding　hydrogen　to　ammonia　increases　the　potential　explosion　hazard　before　considering　the　development　of　ammoniahydrogen　fuel.　In　this　study,　the　effect　of　vent　area　on　vented　deflagrations　of　ammonia-hydrogen-air　mixtures　is　investigated　in　a　1-m-long　horizontal　rectangular　duct　with　a　right　end　opening　at　an　initial　temperature　of　293　K　and　an　initial　pressure　of　101　kPa.　A　dimensionless　vent　coefficient　K-v　is　used　to　define　the　vent　area　in　this　work　to　clarify　the　evolution　of　flame　structures　and　overpressure　inside　and　outside　the　duct.　For　a　specific　K-v,　the　amplitude　of　the　maximum　internal　explosion　overpressure　(P-max)　monitored　at　the　left　end　of　the　duct　(LE)　has　a　greater　difference　compared　with　other　pressure　monitoring　points.　The　maximum　amplitude　of　P(max　)is　always　reached　at　the　LE　for　K-v　＜=　3.2,　while　the　difference　in　the　maximum　amplitude　of　P(max　)between　different　monitoring　points　decreased　significantly　for　K-v　＞　3.2.　With　the　increasing　of　K(v　)from　2.2　to　20.1,　P(max　)obtained　at　the　right　end　of　the　duct　(RE)　and　the　center　of　the　duct　increases　monotonously,　but　P(max　)monitored　at　the　LE　shows　a　non-monotonic　increasing　trend.　A　spike　structure　of　penetrating　flame　bubbles　resulting　from　negative　pressure　balance　is　only　observed　inside　the　duct　for　K-v　＜=　5.6.　Two　types　of　oscillations　(Helmholtz　and　Acoustic　oscillations)　can　be　distinguished　in　some　tests,　and　only　acoustic　oscillations　of　overpressure　can　be　clearly　found　in　tests　with　K-v　＞　7.8.　With　the　increasing　of　K(v　)from　2.2　to　20.1,　the　shape　of　the　external　fireball　will　change　significantly.　As　Kv　increases　from　2.2　to　5.6,　the　external　fireball　shapes　are　always　mushroom-shaped.　However,　the　shapes　of　the　fireball　become　elongated　for　K-v　＞　7.8,　and　even　the　external　fireball　presents　a　jet　shape　for　K-v　=　20.1.　With　the　increasing　of　K-v　from　2.2　to　20.1,　The　maximum　external　overpressure　increases　first,　thereafter　decreases,　and　finally　increases.