Isothermal compression tests of Ti555211 titanium alloy has been conducted on a Gleebe-3800 hot-simulator at the strain rate ranging from 0.001 to 1 s-1 and the deformation temperature ranging from 750 to 950 oC. A constitutive model using dislocation density rate as an internal state variable has been proposed for hot working of Ti555211 titanium alloy. According to the isothermal compression test curves, the influence of deformation temperature and stain rate on stress has been investigated. The results show that the correlation coefficient (R) and the average absolute relative error (AARE) for the constitutive model using dislocation density rate as an internal state variable are found to be 0.94277 and 15.25%, respectively, which provide a guide for process optimization and control of near β deformation of Ti555211 titanium alloy.