The impact of thermal damage on the mechanical properties of rock under high-temperature conditions remains a key scientific issue in geotechnical engineering safety assessment. This study focuses on limestone as the research subject. It employs a combination of systematic physical and mechanical experiments (uniaxial compression and Brazilian splitting) with multi-scale characterization techniques (Scanning Electron Microscopy and Energy-Dispersive Spectroscopy and ultrasonic testing) to comprehensively reveal the evolution of thermal damage and the degradation of mechanical performance in limestone within the range of 25–500°C. The results show that as temperature increased, the porosity of limestone increased from 0.18 % to 1.52 %, the P-wave velocity attenuation rate reached up to 64.7 %, and the elastic modulus, uniaxial compressive strength, and tensile strength decreased by 32.2 %, 45.0 %, and 64.3 %, respectively. Microscopic analysis reveals that thermal damage exhibits a staged behavior. In the low-temperature stage (≤300°C), the damage is primarily caused by intergranular thermal stress-induced microcracks. In the high-temperature stage (≥400°C), partial calcite decomposition (CaCO₃ → CaO + CO₂) occurs, leading to pore network connectivity and ultimately resulting in a coupled “thermal–mechanical–chemical” damage mechanism. Damage variables (D_E and D_P), defined based on elastic modulus and P-wave velocity, demonstrate scale-dependent sensitivity to thermal damage. D_P is more sensitive to the initiation of microcracks, whereas D_E is more effective in assessing high-temperature cementation deterioration. This study provides a critical quantitative foundation for predicting the stability of limestone in deep geothermal development and high-temperature underground engineering.