Thermal-deformation behavior and mechanism of a near-β titanium alloy TC18 in the mono-phase zone were studied by isothermal-compression experiments. The flow behavior of the alloy is predicted by an Arrhenius-constitutive equation considering strain. From the hot-working diagram, the unstable region primarily concentrates in high-strain-rate region, and is dominated by Deformation Bands (DBs) and Adiabatic Shear Bands (ASBs). The numbers of DBs and ASBs increase with temperature, and DBs are the precursors to ASB. Obvious Dynamic Recovery (DRV), Discontinuous Dynamic Recrystallization (DDRX), and Continuous Dynamic Recrystallization (CDRX) exist in the stable region. The predominant microstructural evolution mechanisms at large strains shift from the synergistic effect of CDRX and DDRX below the inflection point temperature and at low strain rate, and the inhibitory effect of DDRX on CDRX above the inflection point temperature and at low strain rate, to the synergistic effect of both at medium strain rate, accompanied by enhanced DRV, and finally to DBs and ASBs at high strain rate. The main microtexture components for the deformed alloy are η_bcc fiber, ε_bcc fiber, α_bcc fiber, γ_bcc fiber, and ξ_bcc fiber. The deformation microtexture is weakened owing to the stochastic orientation distribution of DRX grains.