Superplastic forming is one of effective method to solve the forming difficulty of Ti₃Al based alloy. In this work, the superplasticity of Ti-23Al-17Nb alloy sheet under the conditions of 940~1000 ℃ and 1.7×10^-3~5.5×10^-5 s^-1 are studied. The results show the elongation changes as a parabola with the deformation temperature increasing, and the maximum elongation obtained at 960 ℃ and 5.5×10^-5 s^-1 is 1447.5%. Deformation hardening period increases much more than soften period due to the increasing of element Nb under low strain rate. Compared with primary microstructure, the superplastic deformation could eliminate the texture, the lath-like α₂ grains gradually disappeared, the α₂ grains became more equiaxed, and the content and size of α₂ grains are decreasing with increasing of deformation temperature, the volume fraction of α₂ and B2 phase could reach the optimum deformation at 50∶50. A constitutive relationship based on the Zener-Hollomn parameter and Arrhenius equation was defined for the TAC-1B alloy, and the deformation activation energy Q=390.76 kJ/mol. The results could provide a theory basis for the design and control of TAC-1B alloy superplastic forming process.