The tensile properties of TG6 titanium alloy forging disc as solution and aging treatment (STA) state and STA plus 600 °C, 100 h exposed state were tested at various temperatures. The results show that the tensile strength decreases while the ductility increases with the test temperature increasing. Compared with as STA state, the room temperature tensile plasticity of the specimen subjected to a long-term thermal exposure decreases remarkably. However, once the temperature rises over 150 °C, the tensile ductility increases rapidly, namely, the thermal stability gets a recovery to a great extent. The loss of thermal stability of TG6 titanium alloy may be mainly attributed to the coherent precipitation of a2 phase and surface oxidation. When the temperature is above 150 °C, the resumption of the thermal stability would come from the change of dislocation slip mode. Upon temperature rising, the slip mode of the dislocation changes from concentrated slip mode by cutting the coherent a2 particles to cross slip mode, which will promote a more homogeneous plastic deformation and result in the improvement of tensile ductility, showing the characteristic of the macro-transferring from a planar slip to a wave slip on the fracture surface.