Abstract: To elucidate the damage evolution and instability mechanisms of coal specimens under graded in situ stress conditions, true triaxial single-face unloading tests are conducted at five in situ stress levels using a true triaxial perturbation unloading rock test system in conjunction with acoustic emission (AE) monitoring. The results indicate that, within the tested stress range, the stress–strain curves of the coal specimens exhibit multistage stepwise stress drops, while both the load-bearing capacity and deformation capacity generally increase with increasing in situ stress level. Characteristic stresses are identified using the cumulative AE ringing-count difference method, and the crack evolution process is divided, based on the temporal AE response, into four stages: crack closure, crack development, stable crack propagation, and crack damage propagation. With increasing in situ stress level, the proportion of time spent in the crack damage propagation stage generally decreases. High-energy AE events become increasingly concentrated near the unloading face, while their spatial distribution extends further into the specimen interior, indicating that unloading-induced damage progressively evolves from distributed propagation to localized instability. The b-value and spectral characteristics indicate that crack activity evolves from distributed small-scale cracking toward through-going instability controlled by large-scale cracks under high in situ stress. RA-AF analysis further indicates that the dominant fracture mode progressively shifts from shear-dominated to tensile-dominated. The brittleness index generally increases with increasing in situ stress level. Under high in situ stress, the post-peak load-bearing capacity decreases more rapidly, crack linkage and coalescence occur in a more concentrated manner, and the propensity for sudden instability increases. Under the single-face unloading path, instability of the coal specimens is jointly controlled by unloading-induced differential deformation, coupled tensile–shear action on crack planes, and subsequent vertical loading. As the in situ stress level increases, both the unloading magnitude and the extent of normal compressive stress release on crack planes increase, thereby further enhancing crack opening and localized propagation near the unloading face. Consequently, dominant cracks rapidly coalesce into through-going fractures, ultimately resulting in localized instability. These findings provide a theoretical basis for stability control of surrounding rock in deep roadways and the prevention and control of unloading-induced hazards.