Effects of the isothermal forging deformation amount on the microstructure and properties of the TG6 alloy (Ti-5.8Al-4.0Sn- 4.0Zr-0.7Nb-1.5Ta-0.4Si-0.06C, mass fraction) were studied by optical microscope, SEM, EPMA, metallurgical image-analysis software and the Vikers hardness testing. The results show that with increasing of the deformation amount, both the content of primary a phases and the thickness of secondary-a phases decrease firstly and then increase; the tensile strength increases firstly and then decreases, while the plasticity is contrary. Recovery is dominant for the forging microstructure when the deformation amount is 10%, while for 30% deformation partial dynamic re-crystallization occurs in b phases and recovery is predominant in primary a phases. The lath-shaped a phases caused by the segregation of the Al element for both 10% and 30% deformation can improve the ductility significantly. When the deformation is 50%, the forging microstructure is re-crystallized completely and bimodal microstructure is formed with about 20% primary a phases; so the forging possesses good mechanical properties. Complete re-crystallization happens firstly and partial re-crystallization and recovery appears later when the deformation is 70%; the primary a phases are various and the lath-shaped a phases are caused by the assembly of the lamellar secondary-a phases. The optimal isothermal forging deformation of TG6 alloy is 50% from the analysis of the tensile properties of typical titanium alloys used at 600 °C.