Mixing　helium　and　argon　as　the　working　gas　of　cold　plasma　jet　is　expected　to　combine　the　merits　of　He　plasma　jet　and　Ar　plasma　jet　and　has　great　potential　for　medical　applications.　For　this　reason,　the　discharge　characteristics　and　reactive　species　diagnosis　of　a　He　+　Ar　+　O-2　plasma　jet　were　studied　in　this　paper.　The　result　shows　that　the　Penning　effect　between　He　and　Ar　is　strongest　when　the　volume　fraction　of　Ar　in　the　working　gas　is　10%,　and　the　plasma　jet　propagates　the　longest　distance.　A　low　drive　voltage　and　a　wide　pulse　width　will　weaken　the　Penning　effect　between　He　and　Ar,　resulting　in　a　decrease　in　the　ionization　efficiency　of　the　plasma　jet.　Gelatin　gel　was　used　as　the　model　human　tissue,　and　the　effective　treatment　area　of　He　+　Ar　+　O-2　plasma　jet　on　the　model　tissue　increases　with　the　volume　fraction　of　Ar　in　the　working　gas.　When　the　volume　fraction　of　Ar　is　50%,　the　effective　treatment　area　of　He　+　Ar　+　O-2　plasma　jet　is　9　times　that　of　He　+　O-2　plasma　jet,　and　the　surface　distribution　of　reactive　oxygen　species　on　the　model　tissue　is　almost　uniform.　Moreover,　with　the　increase　of　volume　fraction　of　Ar,　the　influence　of　the　change　of　the　plasma　jet　treatment　angle　on　the　surface　distribution　of　reactive　oxygen　species　is　significantly　weakened,　indicating　that　He　+　Ar　+　O-2　plasma　jet　can　precisely　control　the　reactive　species　dosage　on　the　living　tissue　for　clinical　application.