Abstract: Researching methods for modifying and enhancing the permeability of fractured reservoirs in enhanced geothermal systems (EGS) is of significant strategic importance for promoting efficient thermal energy utilization and achieving green and sustainable energy development. To investigate the permeability enhancement mechanisms of carbon-oxygen thermochemical modification of granite gneiss, Carbon-oxygen thermochemical modification experiments were conducted on single-fracture granitic gneiss at modification temperatures of 225, 250, 275, 300, and 325 ℃. By comparing the fracture seepage parameters and fracture surface morphology before and after modification, the evolution of seepage caused by carbon-oxygen thermochemical modification and its modification mechanism were systematically revealed. Studies have shown that: Carbon-oxygen thermochemical modification technology can significantly improve the permeability of single-fracture granite gneiss, and the permeability of the sample gradually increases with the increase of modification temperature, and shows a significant increase at 275 and 325 ℃. With the increase of volume stress, the permeability growth rate of the sample before and after modification shows an increasing trend. The structural weakening induced by thermochemical action is more likely to promote the connection of fracture surfaces under high volume stress condi-tions, showing the characteristics of “high pressure promoting permeability”. After carbon-oxygen thermochemical modification, the fracture surface tends to be flatter, the elevation fluctuation is reduced, the overall roughness is reduced, joint roughness coefficient (C_JR) decreases by 1.50%–8.27%, and the surface roughness ratio R_s decreases by 0.10%–1.27%. Through fitting, it was found that there is a significant positive correlation between the C_JR reduction rate and the permeability growth rate. Carbon-oxygen thermochemical modification enhances the erosion effect of micro-protrusions on the fracture surface. As the modification temperature increases, the erosion effect of micro-protrusions on the fracture surface gradually evolves from a local and non-uniform distribution to a large-scale and relatively uniform distribution. The area of the elevation zone gradually transforms into the low and medium elevation zones. Carbon-oxygen thermochemical modification can effectively modify reservoir fractures through exothermic reactions under high temperature conditions, thereby achieving reservoir permeability enhancement. Scanning electron microscopy (SEM) results showed that as the modification temperature increased from 225 ℃ to 325 ℃, the microscopic damage characteristics of transgranular fractures, intergranular fractures, particle breakage, and local detachment on the fracture surface gradually increased. The micro-protrusion damage and structural reconstruction induced by carbon-oxygen thermochemical heat release effectively improved the flow space of the fracture surface, thereby improving the conductivity of the fracture. Carbon-oxygen thermochemical modification can effectively transform reservoir fractures through exothermic reactions under high temperature conditions, thereby achieving reservoir permeability enhancement. The research results provide a new idea and technical approach for the efficient diversion and sustainable utilization of fractured reservoirs in enhanced geothermal systems (EGS).