The exploration and development of oil and gas resources have shifted from shallow to deep and ultradeep. The difficulty of rock breaking has also increased, introducing new challenges to traditional rock-breaking technology. Hence, there is an urgent need to develop new rock-breaking technologies to improve the development efficiency of deep oil and gas resources. Therefore, this study focused on the new microwave rock-breaking technology and conducted experimental and numerical simulation research on typical deep, hard rock granite. The research results showed that granite in the microwave field exhibited high-temperature melting and fracture, and the highest temperature could reach 550 °C. Under the irradiation of circulating microwaves, a minimum irradiation time threshold of 3 min was needed to cause irreversible damage to the rock. The numerical simulation results showed that the interaction of thermal stress and in situ stress would cause the inside of the rock stratum to separate into a disturbed deterioration area, disturbed unloading area and initial stress area. These results are expected to provide the necessary technical guidance and theoretical support for the research and development of high-efficiency rock-breaking drilling for deep hard rock.