The changes and pore structure characteristics of high-temperature granite surface cracks under different heat transfer methods were tested by polarizing microscopy (PM), scanning electron microscopy (SEM), and nuclear magnetic resonance imaging (NMR) to analyze the microstructure changes of geothermal reservoir rocks and quantitatively characterize the effective pore characteristics. A fractal dimension model of granite at different temperatures was established to evaluate the storage performance of geothermal reservoirs based on the fractal geometry theory. The granite at different temperatures had significant changes in the types and morphologies of microcracks under different heat transfer methods. Original microcracks closed to decrease porosity at 150°C. There exist grain boundary microcracks at 300°C, intracrystalline microcracks at 450°C, and transgranular microcracks at 600°C with the increased temperature. Meanwhile, the fractal-dimensional thermal storage performance evaluation method of geothermal reservoirs was applied to find the optimal temperature conditions (450 and 600°C) for the thermal storage performance of macropores and mesopores.