To　investigate　the　material＇s　mechanical　appearance　under　complex　stresses　in　the　whole　load　processes,　solid　cylindrical　specimens　are　used　for　the　tension-torsion　tests.　Meanwhile,　how　to　realize　the　fixed　proportional　(C(σ/τ))　tension-torsion　loading　test　in　the　life　cycle　of　solid　cylindrical　shaft　is　derived.　24　series　of　experiments　of　16MnR　carbon　steel　and　304　stainless　steel　specimens　have　been　done　under　different　C　values.　Based　on　test　results,　the　calculation　method　of　equivalent　true　stress　and　strain　of　solid　cylindrical　specimen　is　derived.　Hence,　the　influence　of　different　stress　state　on　the　plastic　limit　strain　is　described　by　triaxial　stress　coefficient　(TS).　Results　show　that:　the　plastic　limit　stress　of　the　two　materials　has　a　minimum　value　through　the　change　of　TS　values,　which　defined　as　stress　stationary　point.　The　plastic　limit　strain　of　the　two　materials　is　in　inverse　proportion　to　TS.　When　the　stress　state　of　a　point　is　determined,　the　plastic　limit　stress　and　strain　can　be　calculated　by　the　formula　presented　in　this　paper.　Difference　of　material　make　the　limit　stress-TS　and　limit　strain-TS　mathematical　models　different,　so　that　the　model　should　be　calibrated　respectively.　©　2017,　The　Editorial　Board　of　Journal　of　Basic　Science　and　Engineering.　All　right　reserved.