Abstract: Due to limitations in high-temperature test equipment, the study of the real thermal-cracking of rocks in laboratory typically involves inverse analysis based on microstructure observations of cooled specimens. Real-time cracking evolution at high temperatures cannot be obtained through this method. Therefore, this study constructs a thermo-mechanical coupled UDEC grain-based model of granite based on the modified joint constitutive law considering temperature and crack slip effects. This is done to investigate the real-time thermal cracking behavior of granite during heating and cooling processes. The study found that thermally induced microcracking in granite began to occur at around 75°C under heating conditions. The number of microcracks rapidly increased near the α→β quartz phase transition temperature, but the microcrack density did not change significantly during the cooling process. Although the change in crack number caused by the cooling effect is negligible, it can lead to an increase or decrease in crack opening. During the heating process, the initiation of microcracks is mainly formed by the local stress accumulation due to the different thermal expansion of adjacent grains. The microstructure changes caused by quartz transition can enhance the interaction between different grains, leading to increased compression and shear motion on the grain level. This results in thermal-induced cracks continuing to deform and develop. During the cooling process, the local microscopic stress release due to thermal cracking during heating and the shrinkage of different mineral crystals due to cooling effects make the number of microcracks hardly to change, but their morphological characteristics can change more significantly. This greatly affects the macroscopic stress-strain behaviors of granite after cooling. The findings of granite thermo-mechanical coupling tests based on discrete element numerical simulations are interpreted in a micro-meso-scale manner，revealing the real-time thermal cracking mechanism of granite during heating and cooling, further promoting the understanding of the thermo-mechanical couplings of high-temperature rocks.