This paper investigates the thermal healing effect of mild temperatures (100–350°C) on the pores of granite. First, a sequence of thermal treatments was conducted on granite at various temperatures: 100°C, 150°C, 200°C, 250°C, 300°C, 350°C, 400°C, and 600°C. Subsequently, nuclear magnetic resonance (NMR) tests were conducted to assess the thermally induced changes in the pore structure of granite by analyzing the T₂ spectrum, magnetic resonance imaging (MRI), porosity, and pore size distribution. Finally, the strengthening factor was introduced to further explore the thermal healing effect of mild temperatures on granite pores. The results show that the porosity of granite first decreases at mild temperatures (100–350°C) and then increases at high temperatures (400°C and 600°C) compared with that observed before thermal treatment, indicating that the mild temperatures can heal the granite pores. The thermal healing effects are further demonstrated by MRI scans of granite before and after thermal treatments. Focusing on the mild temperatures, the strengthening factor initially increases and then decreases with increasing temperature. The thermal healing effect on the granite discussed in this paper is most significant at approximately 250°C. In addition, the thermal healing effect is influenced by pore size. As the temperature rises, the strengthening factor of micropores remains approximately constant, whereas the strengthening factors of mesopores and macropores initially increase and then decrease. In particular, the changes in porosity and the strengthening factor of macropores are the most significant. Moreover, the thermal healing effect of mild temperatures primarily focuses on the macro-porous structure of granite.