Entrainment　occurs　during　the　high-pressure　gas　jet　process,　which　is　crucial　for　a　natural　gas　direct　injection　engine.　This　study　presents　an　experimental　investigation　on　the　high-pressure　methane　jet　from　one　single-hole　injector　and　proposes　a　method　to　obtain　the　entrainment　mass　flow　rate　based　on　kinetic　energy　conservation.　The　entrainment　is　related　to　three　variables,　i.e.,　spring　plate　moving　distance　Delta(x),　gas　jet　mass　at　the　nozzle　outlet　m(n),　and　gas　jet　velocity　u(1).　A　spring-set　test　rig　is　built　to　measure　the　spring　plate　moving　distance　Delta(x),　and　the　schlieren　method　is　adopted　to　test　the　gas　jet　velocity　u(1)　based　on　a　constant-volume　bomb　(CVB)　optical　test　rig;　finally,　the　weight　method　is　used　to　obtain　the　methane　gas　jet　mass　at　the　nozzle　outlet　mn.　This　combined　measuring　method　is　verified　to　be　valid　in　the　near　field　to　the　nozzle.　The　results　show　that　the　methane　jet　mass　flow　rate　gradually　increases　along　the　jet　direction　and　has　a　two-zone　entrainment　process.　Zone　I:　near　field　(Lr　＜　10),　the　methane　jet　mass　flow　rate　linearly　increases　up　to　the　maximum;　in　the　nozzle　exit　field　(Lr　＜　1),　it　is　conserved,　and　no　entrainment　occurs.　Zone　II:　far　field　(Lr　＞=　10),　the　jet　mass　flow　rate　maintains　the　maximum,　and　the　entrainment　becomes　saturated　with　a　saturation　value　larger　than　the　initial　value　at　the　nozzle　outlet.　The　entrainment　rate　experiences　three　stages,　linearly　increasing　in　stage　I　and　early　stage　II　but　not　in　late　stage　II　and　stage　III.　The　methane　injection　pressure　causes　great　effects　on　the　mass　flow　rate　and　entrainment.　As　the　injection　pressure　increases,　the　methane　jet　mass　flow　rate　increases　linearly,　but　the　entrainment　rate　decreases.