Abstract: The thermal deformation behavior of metastable β titanium alloy Ti-4Al-5Mo-6Cr-5V-1Nb was investigated by using dynamic simulator Gleeble-3800 under the condition of deformation temperature from 700 ℃ to 900 ℃ and strain rates from 0.001 s^-1 to 1 s^-1 and strain from 10 % to 50 %. The microstructure evolution and properties of the alloy during thermal deformation process were analyzed. The results show that the flow stress curves of the alloy don’t change much with the increase of strain, the distortion of microstructure in main deformation zone and dynamic recrystallization of deformed crystals increase. With the increase of deformation temperature, the peak stress and steady-state flow stress of the alloy decrease, the microstructure deformation tends to distribute uniformly, and the degree of dynamic recrystallization also increases. Otherwise, due to stress-induced phase transformation, the structure transforms into β phase completely at 790 ℃,which is lower than the transformation temperature slightly. As the strain rate increases, the peak stress and steady-state flow stress increase, the microstructure distortion is more and more inhomogeneous, and the degree of dynamic recrystallization decreases. The alloy is a material sensitive to temperature and strain rate. Its temperature sensitivity increases gradually with the increase of strain rate, and the strain rate sensitivity decreases gradually with increasing temperature.