The free-grained microstructure of TA15 titanium alloy was prepared through two-step forging technology combined with high and low temperatures, and a transnormal superplastic elongation of more than 2000% was obtained. The superplastic behaviour and microstructure evolution were systematically researched at different temperatures and strain rates through superplastic tensile test. The results indicate that the free-grained TA15 alloy exhibits superplasticity at temperatures of 760-980℃ and initial strain rates from 1.1 ×10-2 to 5.5× 10-5s-1.The optimal superplastic conditions are 940℃ and 3.3× 10-4 s-1, in which the average elongation is 2526% and the maximum elongation is 2743%. During superplastic deformation, dynamic recovery and recrystallization occur obviously, and the corporate effect of strain harden-ing and recrystallization softening decides the superplastic ability directly.