Abstract: During the exploitation of deep geothermal energy, the thermal rocks will cool with different cooling rates. A comprehensive understanding of mechanical behavior of the thermal-treated rock is very important for deep underground engineering. However, the fracture behavior and influence mechanism of thermal granite specimens under different cooling ways are unclear at present. Therefore, in this study, the three-point bedding tests are carried out on the semicircular bend granite specimens after high-temperature treatment. The load-displacement curves, fracture toughnesses and failure patterns of the post-heated granite specimens are analyzed, and the evolution laws of micro-cracks and mineral components are discussed. The experimental results show that: (1) As the temperature increases, the fracture toughness of the granite specimens decreases. The fracture toughness of the specimens after quenching in water is lower than that after cooling down naturally in the furnace. (2) The crack initiated from the tip of notch propagates toward the loading point and splits the specimen into two parts. As the temperature increases, the tortuosity degree and deviation of fracture trace of the semicircular bend granite specimens increase. (3) The mineral components of granite are not significantly changed after high-temperature treatment. The micro-crack rate identified by the image processing increases with the increase of high temperature, and that of the specimen after quenching in water is higher than that after cooling down naturally in the furnace, which indicates that the deterioration of micro-structure of rock induced by high-temperature treatment reduces the fracture toughness of granite.