Stress relaxation tests were performed on Ti-6.5Al-2Zr-1Mo-1V titanium alloys with different initial stresses at 500, 550, and 600 °C. Based on the classical Maxwell exponential decay function, the stress relaxation limit was obtained. The relaxation stability coefficient (C_S) and relaxation rate coefficient (C_R) were proposed to describe the relaxation characteristics and to further guide the residual stress reduction. The stress exponent was calculated according to Norton and Arrhenius equations. Both the stress exponent and microstructure were analyzed to illustrate the stress relaxation mechanism. Under different initial stresses, the dislocation climb and diffusion dominate the stress relaxation procedure at 500 °C; the dislocation slip plays a major role in stress relaxation at 550 °C; the dislocation slip, boundary slip, and grain rotation control the stress relaxation process at 600 °C.