By means of the uniaxial isothermal compression test, the thermal deformation behaviors of laser additive manufactured TC18 titanium alloys are studied under different thermal deformation conditions. The regularity of flow stress-strain and softening mechanism are analyzed. The constitutive equation of peak stress is established. The results show that the flow stress-strain curves of laser additive manufactured TC18 titanium alloys can be characterized by two kinds of characteristics of the continuous softening and the steady rheology, and the activation energy is 476.8 kJ·mol-1. When the thermal working temperature is in the α+β two-phase region, the softening mechanism is mainly the dynamic recovery; when the thermal working temperature is in the β single phase region, the softening mechanism is the dynamic recrystallization. The ideal thermal process parameters for laser additive manufactured TC18 titanium alloys are the defomation temperature of 830-880 ℃ and the strain rate of 0.001-0.003 s-1, or the deformation temperature of 750-760 ℃ and the strain rate of 0.001-0.002 s-1.