Direct　injection　in　the　cylinder　is　a　key　technique　for　natural　gas　engines.　The　characteristics　of　gas　jet　and　its　entrainment　are　important　for　the　optimization　of　the　direct　injection　natural　gas　engine.　The　high-pressure　methane　jet　flow　of　a　circular　nozzle　was　studied　by　simulation　based　on　a　three-dimensional　model.　The　simulation　results　such　as　the　mass　flow　rate　and　the　tip　penetration　were　verified　by　experimental　data.　Based　on　the　numerical　model,　the　jet　flow　characteristics　were　investigated　under　injection　pressures　of　5,　10　and　20　MPa　with　a　constant　back　pressure　of　2MPa.　The　results　show　that　the　gas　velocity　appears　saturation　at　the　outlet　of　the　nozzle　since　the　gas　jet　velocity　reaches　the　sound　velocity.　In　addition,　the　gas　jet　flow　velocity　has　a　two-region　feature.　In　the　near　field,　a　Mach　disk　is　induced　by　rapid　gas　expansion　when　the　gas　just　leaves　the　nozzle,　resulting　in　a　sharp　decline　of　the　gas　jet　velocity.　The　natural　gas　jet　flow　has　a　strong　ability　to　entrain　the　background　gas,　thus　the　mass　flow　rate　of　gas　jet　is　greater.　In　the　far　field,　since　the　Mach　disk　disappears,　the　gas　expansion　effect　is　weak　and　the　gas　jet　velocity　becomes　stable.　The　entrainment　capacity　of　natural　gas　jet　flow　is　also　weakened,　and　the　mass　flow　rate　is　basically　unchanged.　These　characteristics　of　high-pressure　methane　jet　flow　and　its　entrainment　are　helpful　to　understand　the　mixing　process　of　the　natural　gas　direct　injection　engine.　©　2020,　Editorial　Office　of　the　Transaction　of　CSICE.　All　right　reserved.