Oil-sand reservoirs saturated with heavy oil are subject to complex physical and chemical changes under high-temperature conditions. These changes can be quantified using the thermal damage factor, which we evaluated based on Young’s modulus and the velocities of P- and S-waves. For the quasi-solid phase of heavy oil-bearing rocks, this method effectively characterizes the degree of rock damage. As temperature increases, heavy oil transitions into a fluid state, reducing rock stiffness. In addition, the thermal expansion of heavy oil weakens the rock matrix and influences the extent of rock damage. We combined an extended Gassmann equation with the Maxwell model for heavy oil to estimate the thermal damage factor. The model was validated using ultrasonic experimental data from rock samples, enabling a quantitative description of the relationship between dry and wet rocks at different temperatures of thermal damage in oil sand. We found that in these rock samples, the temperature-dependent trend of the thermal damage factors can be separated into two stages based on the fluid-viscosity threshold (the liquid point). The porosity of rock samples has no significant influence on this threshold, whereas the viscosities of different fluids affect the threshold value of the thermal damage factor in oil sands. The proposed model provides a theoretical basis for improving the accuracy of reservoir prediction, evaluation, and adjustment, and for optimizing heavy-oil thermal recovery. Furthermore, it offers practical applicability for thermal-recovery monitoring and numerical simulation, enabling more reliable interpretations of temperature-dependent elastic responses.