Abstract: Annealing treatment is an important technology to improve the microstructure, mechanical properties, and superplastic properties of titanium alloy. Forged Ti-6Al-4V alloy was annealed at the temperature of 850℃-950℃ in a vacuum annealing furnace, and the superplastic tensile tests of Ti-6Al-4V alloy were conducted at a temperature of 900℃ and strain rate of 0.01 s^-1. The effect of high-temperature vacuum annealing on microstructure evolution and superplasticity of Ti-6Al-4V alloy was investigated. The results show that the primary α grain size decreases with the increase of vacuum annealing temperature, and the proportion of β phase increases with the vacuum annealing temperature. The distribution of grain size and phase of α and β is relatively uniform in the sample annealed at 910℃, which exhibits the best superplastic tensile performance with the maximum tensile elongation of 785% and minimum peak stress of 26.8 MPa.