The isothermal compression experiment of as-rolled Ti55 alloy was carried out on a Gleeble-3800 thermal simulation test machine at the deformation temperature range of 700–1050 °C and strain rate range of 0.001–1 s⁻¹. The hot deformation behavior and the microstructure evolution were analyzed during thermal compression. The results show that the apparent activation energy Q in α+β dual-phase region and β single-phase region were calculated to be 453.00 KJ/mol and 279.88 KJ/mol, respectively. The deformation softening mechanism was mainly controlled by dynamic recrystallization of α phase and dynamic recovery of β phase. Discontinuous yielding behavior mainly occurred in β phase region, which weakened gradually with the increase of deformation temperature (>990 °C) and strain rate (0.01–1 s⁻¹) in β phase region. The processing map derived from Murty’s criterion was more accurate in predicting the hot workability than that derived from Prasad’s criterion. The optimized hot working window was 850–975°C/0.001–1 s⁻¹, in which sufficient dynamic recrystallization occurred and α + β-transus microstructure was obtained. When deformed at higher temperature (≥1000 °C), coarsened lath-shape β-transus microstructure was formed, while deformed at lower temperature (≤825 °C) and higher strain rate (≥0.1 s⁻¹), the dynamic recrystallization was not sufficient, thus flow instability appeared because of shear cracking.