Metastable high-entropy alloys can exhibit coupled phase transformation, strain localization and damage evolution under shock loading, and their spall responses are governed by both the loading intensity and the duration of the stress pulse. To clarify the distinct roles of impact velocity and pulse duration in spallation behavior and void evolution, plate-impact experiments were conducted on a Si₄V₅Mn₅Cr₁₀Co₃₀Fe₄₆ metastable high-entropy alloy using a single-stage light-gas gun. The free-surface velocity response, spall parameters, local microstructural evolution and three-dimensional void morphology under different loading conditions were systematically investigated. The results show that, at a fixed specimen thickness, increasing the impact velocity from 282 m/s to 553 m/s markedly raises the peak free-surface response and the peak compressive stress, whereas the spall strength only slightly changes. Meanwhile, internal voids evolve from dispersed nucleation to localized clustering, accompanied by increased fractions of HCP/BCC (hexagonal close-packed/body-centered cubic) phases and a pronounced rise in high-KAM (kernel average misorientation) regions near the voids, suggesting that higher impact velocity promotes local phase transformation, lattice distortion and concentrated damage development. Micro-X-ray computed tomography further reveals that increasing impact velocity drives the voids towards larger volumes, stronger spatial concentration and more complex morphologies. In contrast, under nearly constant impact velocity, as the specimen thickness increases from 1.0 mm to 2.0 mm, the pulse duration is prolonged from 0.075 μs to 0.250 μs, and the spall strength correspondingly increases from 1.40 GPa to 1.83 GPa. The spatial distribution of voids gradually changes from dispersed to centrally concentrated, with an enhanced tendency for interconnection, indicating that longer pulse duration demonstrates greater favorability towards damage accumulation and localized coalescence. By combining free-surface velocity histories, two-dimensional microstructural characterization and three-dimensional void statistics, it is shown that the impact velocity mainly controls the instantaneous driving force for the damage during spallation, whereas the pulse duration primarily governs the time window for the damage accumulation and the extent of localization. Together, these two factors determine the nucleation sites, growth paths and final failure mode of spall damage in this metastable high-entropy alloy.