The stress-strain curves obtained by thermal simulation compression experiments show that the powder TC4 titanium alloy has the characteristics of work hardening and continuous dynamic softening during deformation at the temperature of 850~950℃ and the strain rate of 0.1~10s_^-1. The constitutive equation of the material was established, which well describes the rheological behavior of the powder TC4 titanium alloy. Furthermore, the dynamic softening behavior was analyzed, and the influence degree of various factors on softening was calculated. The results show that with the decrease of deformation temperature and the strain rate, the flow softening degree increases. When the strain rate is 1s_^-1 and 10s_^-1, the flow softening is mainly caused by the deformation heat. When the strain rate is 0.1s_^-1 and the temperature are 850 and 900℃, there are three softening factors: deformation heat, dynamic phase transformation and α phase morphology evolution. As temperature decreases, the softening proportion caused by morphological evolution of αphase increases. With the increase of temperature, the softening proportion of dynamic phase transformation increases. When the strain rate is 0.1s_^-1 and the deformation temperature is 950℃, there are two softening factors : deformation heat and dynamic phase transformation. With the increase of deformation, the proportion of dynamic phase transformation softening increases.