Abstract: Weakly cemented siltstone is sensitive to cyclic disturbances because of its loose particle structure, developed pores, and weak intergranular cementation. Under repeated loading, this type of rock easily experiences irreversible deformation accumulation, crack propagation, strain localization, and progressive failure. However, the evolutionary relationship between surface nonuniform deformation observed by digital image correlation (DIC) and internal crack activity detected by acoustic emission (AE) remains unclear. In this study, weakly cemented argillaceous quartz siltstone was tested under post-cyclic monotonic loading. Based on the characteristic stresses obtained from uniaxial compression tests and the fatigue-failure boundary determined from preliminary cyclic tests, cyclic upper-limit loads of 45, 55, and 65 kN were selected to represent loading levels below, close to, and above the fatigue-failure boundary, respectively. Three-dimensional DIC and AE monitoring were combined to investigate the strength response, full-field strain evolution, nonuniform deformation characteristics, and crack-type transformation of the specimens after different numbers of cycles. The results show that the cyclic upper-limit load controls the transition of weakly cemented siltstone from compaction-induced strengthening to damage-dominated weakening. When the upper-limit load was below or close to the fatigue-failure boundary, the specimens showed low-cycle weakening and high-cycle strengthening, indicating the combined effects of damage accumulation, pore closure, particle rearrangement, and compaction interlocking. When the upper-limit load exceeded the fatigue-failure boundary, crack propagation and damage localization became dominant. In this condition, the peak strength decreased continuously with increasing cycle number, whereas the peak strain first decreased and then increased. DIC results indicate that high-level cyclic disturbance significantly promoted residual shear deformation accumulation and accelerated strain localization during subsequent monotonic loading. After 100 cycles at the 65 kN upper-limit load, the maximum shear strain during reloading increased by approximately 2.75 × 10⁻³ compared with that during the initial monotonic loading process, and high-strain regions appeared earlier with a higher degree of concentration. To quantify the nonuniform evolution of the DIC strain field, a multi-indicator characterization framework was established using strain standard deviation, high-strain area ratio (HsAR), and nonuniformity index H_j. These indicators characterize the dispersion of strain values, the area proportion of high-strain regions, and the spatial clustering degree of localized deformation, respectively. For the specimen subjected to 100 cycles at 65 kN, the normalized strain standard deviation and HsAR increase from approximately 0.06 and 0.01, respectively, to nearly 0.99, indicating continuous expansion of high-strain regions. The H_j index remains near 0.40–0.50 during the early cyclic stage, but rapidly increased to nearly 1.0 after approximately 75 cycles, suggesting that scattered high-strain zones gradually connect and evolve into a continuous localized deformation band. AE–DIC joint analysis further shows that internal microcrack activity generally preceded observable surface strain localization. The AE ring count reached a high level before the rapid increase in H_j, indicating that microcrack initiation, propagation, and frictional sliding drive the subsequent redistribution of the surface strain field. RA–AF analysis shows that the proportion of shear cracks increased from 79.81% to 99.73% under high cyclic loading, confirming the transition toward shear-dominated failure. These findings reveal the progressive instability process of weakly cemented siltstone under cyclic disturbance and provide a quantitative basis for identifying strain localization and failure precursors in weakly cemented rocks.