Based　on　the　Miedema　formation　enthalpy　model,　thermodynamic　properties　of　ternary　alloy　were　calculated.　Using　the　composition　characteristics　of　the　alloy,　the　thermodynamic　properties　between　elements　of　the　quaternary　alloy　Ti-46Al-8Nb-0.1Er　were　studied.　Combining　Toop　model,　we　derived　alloy　component　activity　and　activity　coefficient.　The　results　show　that　in　this　quaternary　system　Er　only　reacts　with　Al　generating　the　Er-Al　compounds　while　no　reaction　with　the　other　elements　is　obseryed.　Due　to　forming　Er2O3　more　easily　than　Er-Al　comounds　in　thermodynamic,　and　the　existance　of　trace　oxygen　is　inevitable　in　the　molten　alloy,　Er　combined　with　O　forms　Er2O3　particles,　which　are　observed　in　experiments　by　XRD　analysis.　The　Er　addition　has　the　effects　of　purification　of　the　alloy　melt,　reducing　oxygen　content　of　the　alloy　matrix　and　the　precipitation　strengthening.