Abstract: Existing studies on alkali-activated ultra-high-performance concrete (A-UHPC) exposed to elevated temperatures have predominantly focused on residual strength indices, while a systematic understanding of its full-field damage evolution and crack-dominant mechanisms under external loading remains limited. In this study, A-UHPC specimens were subjected to thermal exposure from 25 to 800℃, followed by uniaxial compression, Brazilian splitting, and three-point bending tests. A multi-scale characterization framework integrating acoustic emission (AE), digital image correlation (DIC), and scanning electron microscopy (SEM) was employed to investigate the evolution of mechanical performance, strain localization, and crack mechanisms. The results indicate that the mechanical properties of A-UHPC exhibit a stage-wise degradation pattern with increasing temperature. A pronounced transition from gradual deterioration to significant degradation was observed around 600℃, at which the residual compressive strength decreased to approximately 30% of its original value, while the fracture energy reduced to about 36% at 800℃. DIC analysis revealed a non-monotonic evolution of strain localization under uniaxial compression and three-point bending. At 600℃, strain localization bands emerged earlier, at approximately 0.4Pmax (uniaxial compression) and 0.6Pmax (three-point bending), whereas at 800℃ they reappeared near the peak or post-peak stage. In contrast, under Brazilian splitting, the initiation timing of strain localization remained relatively stable, although noticeable deflection and widening of localization bands occurred between 600 and 800℃.AE-based statistical analysis further demonstrated load-dependent and temperature-dependent variations in crack-dominant mechanisms. Under uniaxial compression, the crack mode gradually shifted from tension-dominated to shear-dominated behavior, with the proportion of shear cracks increasing from 13.7% at room temperature to 55.1% at 800℃. In Brazilian splitting tests, the proportion of shear cracks increased to 43.3% at 800℃, although tensile cracks remained predominant. For three-point bending, crack evolution exhibited a non-monotonic trend: shear cracks reached a maximum proportion of 67.4% at 600℃, while tensile cracks regained dominance (55.6%) at 800℃. SEM observations revealed progressive matrix densification loss and pore connectivity development, accompanied by significant degradation of the steel fiber–matrix interfacial transition zone. This microstructural deterioration is consistent with the observed reduction in mechanical performance and the transition in crack-dominant mechanisms. Overall, this study elucidates the temperature-dependent evolution of damage modes and crack-governing mechanisms in A-UHPC after high-temperature exposure, providing a multi-parameter framework for post-fire structural performance assessment.