To investigate the high-temperature mechanical response and deformation mechanisms of novel high-strength and high-toughness TC21 titanium alloy thin sheets with different initial microstructures, 1.5-mm-thick sheets featuring lamellar and duplex microstructures were utilized to conduct high-temperature tensile experiments. Microstructural characterization of the samples under various deformation conditions was performed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results reveal that the alloy exhibits superplastic deformation characteristics at 880 and 900 ℃ with strain rates of 0.01 and 0.001 s-1,resulting in elongations exceeding 100%. During deformation, dynamic recrystallization and phase transformation exhibit a significant synergistic effect, manifested as grain refinement, pronounced equiaxialization, and a maximum β-phase content.