<p>In the design and excavation process of underground engineering in active tectonic areas, accurately revealing the energy evolution and damage accumulation of surrounding rock is of great significance for preventing and controlling engineering instability disasters. To investigate the progressive damage mechanism of carbonaceous slate under real unloading paths, conventional uniaxial and triaxial, single unloading, Brazilian splitting, and three-point bending tests were performed on rock samples from a railway tunnel crossing the Yalahe active fault section. Meanwhile, based on the differences in damage accumulation calculations between the real unloading path and the assumed unloading path, a damage constitutive model was proposed considering the damage accumulation along the full stress path. On this basis, the mechanical parameters of the surrounding rock mass were calibrated using the monitoring results of the mechanical behavior of the supporting structure, and the engineering applicability of the proposed damage constitutive model was validated. The results indicated that the total energy, elastic strain energy, and dissipated energy all increased exponentially with increasing confining pressure. Additionally, the elastic energy ratio at the peak strength point also increased, reaching values of 0.37, 0.48, 0.66, and 0.73, respectively. Based on the assumed unloading path, the calculated dissipated energy was relatively low, resulting in an overestimation of the damage coefficient by approximately 0.10 to 0.14. Example analysis showed that considering damage accumulation only after the damage threshold led to an underestimation of the damage factor by about 5% to 10%. The applicability of the proposed full stress path damage constitutive model had been verified through both indoor experiments and on-site mechanical monitoring.</p>