This study investigates the deformation mechanisms of slopes and the activation effects within rock masses induced by cyclic deepening and superimposed mining in combined surface and underground operations. Using the Fushun coal field as a case study, a three-dimensional numerical model was established in FLAC 3D to analyze ground surface displacement and slope deformation under multi-source composite mining conditions. The results indicate that: (1) At the end of the single open-pit mining stage (Step 2), the maximum surface settlement and horizontal displacement reached 80 mm and 77 mm, respectively. The values in Step 1 were 52 mm and 21 mm. (2) The maximum settlement increased significantly to 1310 mm, and displacement rose to 357 mm during the concurrent open-pit and underground mining stage (Step 3), attributable to stress superposition effects. (3) In the subsequent open-pit deepening stage (Step 4 to Step 5), cumulative settlement and displacement reached 1520 mm and 491 mm, respectively. The stability of the open-pit slope progressively decreased with mining depth, as reflected by the safety factor declining from 3.23 (Step 1) to 1.69 (Step 5). Displacement vectors on the critical slip surface exhibited spatial differentiation, with the lower mountain boundary area emerging as a high-risk zone for landslides due to the isotropic superposition of displacement vectors. These findings systematically reveal a chain failure mechanism characterized by “stress activation – displacement superposition – slip evolution,” providing theoretical support for disaster prevention and control in mining areas.