<p>Exploring the mechanical behavior of deep rocks, determining the development laws of microcracks, and enriching the evaluation indicators of rock damage are prerequisites for ensuring the safety and stability of underground engineering. In this research, by carrying out triaxial experiments and based on mechanical analysis, acoustic emission (AE) damage characteristics study and Three-Dimensional Particle Flow Code numerical simulation, it is determined that confining pressure can limit internal crack propagation, delay the formation of macroscopic failure surface, reduce damage accumulation rate, and strengthen rock mechanical properties. The development of microcracks inside rocks presents a process of “local aggregation of microcracks to form damage area, continuous development of microcracks to expand damage area, penetration of damage zone to form fracture zone, penetration of fracture zone to form macroscopic failure surface”, which is the fundamental cause of rock damage, and the fracture zone formed by the accumulation of AE high-energy events is the main controlling factor for the formation of rock macroscopic failure surfaces. As quantitative evaluation indicators of rock damage status, AE <i>S</i> value and AE <i>Q</i>_t value are introduced, which can correspond to various stages of rock crack development. The research results can provide some benefits for the safety and stability of deep underground engineering.</p>