<p>The mitigation of unwanted fluid leakage through fracture networks in underground rock reservoirs using microbially induced calcium carbonate precipitation (MICP) technology has been widely recognized as a promising approach. While the influence of thermal damage defect scale on the mechanical properties of MICP-healed granite fractures under various high-temperature conditions remains poorly understood, this knowledge is crucial for ensuring and predicting the long-term effectiveness of MICP healing. By varying sample geometry across the microscale to centimeter scale (μm–mm–cm), the influence of multiscale defects spanning nanometers to centimeters (nm–μm–mm–cm) can be effectively captured using nano-CT (SSRF) and μ-CT techniques. A wide range of temperatures (from 30 ℃ to 1000 ℃) capable of inducing thermal damage defects across multiple scales was studied. Based on a set of self-developed micro-engineering geomechanical testing system, multi-scale compression tests were conducted. As a result, it was found that 500 ℃ appeared as the critical temperature, with the defects larger than 440&#xa0;μm begin to dominate, and the orientation of more defects shifts from horizontal to vertical orientation. Shear, cyclic friction, and nanoindentation tests were performed to facilitate the estimation of the static friction coefficient and to establish a sliding wing crack model. An in-situ non-destructive strength prediction model was developed based on defect characteristics and material parameters, incorporating a disorder index (<i>I</i>_d) and aspect ratio of defects. This model provides a scientific basis for the widespread application of rock and rock-like materials in high-temperature geotechnical environments.</p>