As　the　composition　elements　in　multi-principal　element　alloys　increase,　it　can　bring　excellent　mechanical　properties.　However,　the　strengthening　mechanism　of　the　additional　element　is　still　unclear.　In　this　work,　we　establish　a　method　based　on　the　in-situ　EBSD　technology　to　explore　the　possible　effect　of　additional　elements　on　the　intrinsic　strength　of　tensile　properties.　We　prepared　four　different　multi-principal　element　alloys,　including　FeCoNi,　FeCoNiMn,　FeCoNiCr,　and　FeCoNiCrMn　with　similar　initial　status.　We　systematically　investigated　the　evolution　of　the　microstructure,　dislocation　density,　twin　boundary,　grain　size,　and　element　distribution　during　the　tensile　process　by　in-situ　EBSD　and　EDS.　By　carefully　analyzing　the　results　of　four　different　multi-principal　element　alloys,　the　strength　effects　of　the　solid-solution　hardening,　grain-boundary　hardening,　twin　boundary　hardening,　precipitate　hardening,　and　dislocation　hardening　were　peeled.　The　effect　of　the　Cr　and　Mn　element　addition　on　the　intrinsic　strength　can　be　explored.　It　is　found　that　the　element　addition　indeed　increases　the　intrinsic　strength　from　quaternary　to　quinary　but　not　very　clear　from　ternary　to　quaternary　no　matter　Cr　or　Mn,　which　indicated　that　the　intrinsic　strength　was　more　related　to　the　number　of　elements　in　the　alloy　than　to　which　element　was　present.　This　can　be　explained　using　the　mixing　entropy　theory,　which　states　that　the　intrinsic　strength　is　enhanced　when　the　mixing　entropy　is　over　a　threshold　between　the　MEA　and　HEA.　This　paper　presents　a　method　to　study　the　individual　factors　affecting　the　tensile　properties,　which　can　help　other　researchers　to　better　investigate　HEA.