Based　on　the　software　of　Ansys　Fluent，a　three　dimensional　numerical　simulation　of　ammonia　gas　jet　was　carried　out　to　study　the　characteristics　of　jet　penetration　and　ambient　air　entrainment.　Both　the　under-expended　area　of　the　ammonia　gas　jet　and　the　vortex　structure　were　analyzed.　The　results　show　that　the　development　of　ammonia　jet　penetration　presents　a　three-staged　pattern，which　has　a　strong　correlation　with　jet　formed　initial　vortex　and　shear　layer.　The　first　stage　(0＜t＜0.14　ms)　is　the　linear　stage　where　ambient　air　is　hardly　entrained　at　jet　boundary　and　under-expanded　area.　The　second　stage　(0.14　ms≤t≤0.40　ms)　is　the　transient　stage　where　initial　vortex　and　shear　layer　have　a　rapid　development　while　the　mass　of　entrained　air　increases　over　50%　(R＝8.0).　The　third　stage　(t＞0.40　ms)　is　the　quasi-steady　stage　where　the　maximum　mass　of　entrained　air　decreases　by　30%　(R＝8.0).　Average　mass　of　entrained　air　increases　with　the　increase　of　the　nozzle　pressure　ratio　R.　The　average　transient　mass　of　entrained　air　increases　when　the　initial　vortex　exists　at　the　first　and　second　stage，peaks　at　which　the　shear　layer　fully　develops，where　mass　of　entrained　air　increases　to　58%　(R＝8.0).　©　2024　Chinese　Society　for　Internal　Combustion　Engines.　All　rights　reserved.