Abstract: The dynamic instability mechanism and constitutive characterization of highly stressed sandstone under low-frequency disturbance provide an important theoretical basis for evaluating the stability of interburden strata and preventing dynamic disasters during residual coal mining. Using a self-developed electrohydraulic servo testing system for coupled static–dynamic loading, uniaxial compression tests were conducted on sandstone specimens subjected to a static prestress level of 80% UCS and 5 Hz sinusoidal disturbances with amplitudes ranging from 10% to 30% UCS. The results show that highly stressed sandstone exhibits a pronounced amplitude-threshold effect under low-frequency disturbance. A disturbance amplitude of 10% UCS did not induce dynamic failure within 300 loading cycles; however, disturbance-induced damage accumulated progressively and resulted in a 4.13% reduction in post-disturbance strength relative to the purely statically loaded specimens. When the disturbance amplitude reached 15% UCS or higher, dynamic failure occurred during disturbance loading, and the number of cycles to failure decreased markedly with increasing amplitude. The spatiotemporal distribution of acoustic emission (AE) events further shows that low-amplitude disturbance mainly activates pre-existing defects and local weak structures. Once the disturbance amplitude exceeded the dynamic-failure threshold, high-energy AE events progressively extended from the specimen ends toward the central region and eventually formed a through-going fracture band corresponding to the macroscopic failure pattern. Energy analysis indicates that the strain-energy storage limits before dynamic failure for the 15%, 20%, and 25% UCS groups were 416.93, 424.80, and 417.54 kJ·m?3, respectively, with a coefficient of variation of only 1.04%. This limited variation suggests that the pre-failure critical energy-storage level remained relatively stable within the tested disturbance-amplitude range. The energy evolution can therefore be summarized as high initial energy storage under static prestress, progressive energy accumulation during cyclic disturbance, and final triggering by the last disturbance cycle. The median strain of the dynamically failed specimens exhibited an inverse-S-shaped three-stage evolution, comprising initial rapid adjustment, intermediate stable accumulation, and final accelerated instability. This evolution can be effectively characterized by an inverse Logistic-type function. Accordingly, an inverse Logistic-type correction term was incorporated into the standard Poynting–Thomson model to establish a modified constitutive model that simultaneously captures the periodic viscoelastic response and nonlinear irreversible strain accumulation during disturbance loading. The proposed model reproduces the measured strain evolution under different disturbance amplitudes and quantitatively describes the transition from damage accumulation to dynamic instability. These findings clarify the amplitude-controlled failure response, energy-triggering process, and strain-evolution characteristics of highly stressed sandstone, and provide a theoretical basis for evaluating the dynamic stability of interburden strata under residual coal mining conditions.