Rheological property is one of the deformation characteristics of rock; and great importance shall be attached to the long?term strength of the rock mass among mechanical properties in most geotechnical engineering. According to the triaxial creep test of limestone after high temperature, a thermal damage factor based on longitudinal wave velocity is introduced to quantify the thermal effect on the rock mass, and a series of observations are used including isochronal curves of strain method, transient creep method and steady creep rate method to predict the long-term strength of limestone and put forward an improved method of steady–state creep rate curve-tangent method. The results show that the thermal damage factor which based on wave velocity can quantify the thermal effect accurately; and the thermal damage factor shows a close correlation with the long-term strength. The limestone threshold temperature is 380 ℃ determined in this tests; lower than which the property of limestone shows a reinforcement effect and beyond which shows a damage effect. The steady-state creep rate curve-tangent method predicting the long-term strength of limestone is much accurate and consistent with the actual situation. The failure modes of limestone at different temperatures are characterized by shear failure; and the sub-crack transforms gradually from the parallel to the main shear plane to the irregular extension.