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In order to analyze the change rule of the pore structure of different thermal treated granites and quantitatively characterize the distribution characteristics of effective pores, the pore structure characteristics of high-temperature granites under different cooling modes are tested based on nuclear magnetic resonance (NMR) technology. It is found that when the temperature is 150 °C, water-cooling will reduce the number of rock pores, and the phenomenon of “thermal hardening” is obvious. When the temperature is 300 °C, the micropore diameter increases and part of it changes into mesopore pores. When the temperature is 450 °C, the mesopore pores occupy the dominant position, and the pore types are relatively concentrated. When the temperature is 600 °C, exceeding the quartz phase change temperature (573 °C), the number and pore diameter of rock pores increase dramatically, and even macropores appear. The pore size of rocks under different heat treatments has fractal characteristics, and fractal dimension models related to pore size can be established. Based on the fractal dimension model, the thermal storage properties of different thermal treated granites are discussed, and the optimal temperature conditions (450 °C and 600 °C) for the thermal storage properties of macroporous and mesoporous pores are found.