The　thermodynamic　character　of　phase　transformation　of　TC4　titanium　alloy　electron　beam　welded　joint　was　described.　In　the　point　of　thermodynamic　view,　the　formation　mechanism　of　different　microstructures　of　TC4　titanium　alloy　joints　in　the　different　heat　treatment　conditions　was　analyzed,　which　provides　theoretic　base　for　the　method　to　control　phase　transformation　of　TC4　titanium　alloy　electron　beam　welded　joint　by　changing　heat　treatment　conditions.　The　driving　force　of　phase　transformation　of　TC4　titanium　alloy　electron　beam　welded　joint　comes　from　the　difference　of　chemical　free　energy　between　the　new　phase　martensite　and　the　parent　phase.　The　martensite　forms　throughout　the　grain,　and　its　orientation　was　a　certain　angle.　In　the　case　of　the　post-weld　heat　treatment　which　temperature　is　lower　than　MS　points,　martensite　only　grows　up,　but　in　the　case　of　the　post-weld　heat　treatment　which　temperature　is　higher　than　MS　point,　not　only　martensite　grows,　but　also　part　of　the　β-phase　becomes　saturated　solid　solution　and　leaves　among　the　martensite　film　layers.