Abstract: The ophiolite massif in the southeastern Tibet Yarlung Zangbo suture zone is subjected to high-temperature conditions due to deep geological processes and engineering thermal effects, with its alteration characteristics directly influencing the mechanical behavior of the rock mass. Through a combination of macroscopic mechanical tests and microscopic analysis, the damage mechanism of ophiolite under high-temperature alteration was investigated. The results indicate that high temperatures significantly degrade the mechanical properties of the rock, with tensile strength exhibiting linear attenuation in the 200–400℃ and 600–800℃ intervals, and a reduction exceeding 80% at 1000℃. Microscopic analysis reveals that 600–800℃ is the critical temperature range for the dehydration and decomposition of hydrous minerals, during which porosity increases by over 47% and crack networks develop extensively. XRD and SEM analyses demonstrate that chlorite and muscovite decompose and disappear at 800℃ and 1000℃, respectively, with their dehydration-induced phase transitions governing structural instability. Additionally, the recrystallization and amorphous melting of quartz above 800℃ are identified as the microscopic triggers for strength collapse. This study elucidates the progressive failure mechanism of ophiolite, characterized by “brittle fracture–thermochemical damage–melting failure,” providing theoretical support for understanding the mechanical evolution of ophiolite in tectonic suture zones.