<p>Microwave-liquid nitrogen treatment has attracted huge attention due to its significant damage to rock and broad application prospect in assisting mechanical rock-breaking. Meanwhile, the mode I fracture of hard rock plays a pivotal role in the deep drilling of tunneling boring machines (TBM). Therefore, the structure damage and mode I fracture behaviors of the granite treated with different microwave and liquid nitrogen conditions are analyzed experimentally. The damage mechanism of the microwave-liquid nitrogen treatment is deeply explored and the fracture propagation mechanism of the treated granite under the three-point bending (TPB) loading is given. The results show that the microwave-liquid nitrogen treatment can result in more microcracks and a decrease in the fracture toughness of the granite. The decrease in the fracture toughness of the granite is more obvious after the multiple cycle treatment (after 20 cycles, the decline rate reaches 82.2%). At the same power, the effect of the microwave-liquid nitrogen treatment on the mechanical degradation of the granite is more obvious than that of a single microwave treatment. In the TPB test, the microwave-liquid nitrogen treatment can significantly promote the propagation of shear cracks, especially under the high-cycle microwave-liquid nitrogen treatment. With the increase in the microwave-liquid nitrogen cycle, the required fracture energy decreases, and the induced crack surface is coarser. Under the high-cycle microwave-liquid nitrogen treatment, the granite exhibits ductility and the top loading point is prone to disintegration. Therefore, the microwave-liquid nitrogen treatment can improve the TBM rock-breaking efficiency by decreasing the fracture resistance and fracture energy of hard rock. The results will help to deepen the understanding of the application of microwave-liquid nitrogen treatment in assisting mechanical rock-breaking.</p>