Soil erosion in alpine ecosystems threatens biodiversity, ecosystem services, and SDGs, yet its spatiotemporal dynamics and drivers remain unclear. Using multi-source remote sensing (1990-2022), deep learning, and Random Forest, we assessed soil erosion in Qilian Mountain National Park, an ecotone between the Qinghai-Tibet Plateau and northwestern arid regions, offering a natural laboratory for advancing understanding of water erosion in fragile alpine-arid ecosystems. Results show a mean annual erosion of 2.77 x 10(2) tha(-1)yr(-1) across the whole national park. Over the past three decades, the conversion of bare land to vegetated ecosystems (5355 km(2)) has reduced soil erosion by approximately 5.36 x 10(8) t. Bare land had the highest annual mean erosion (100.17 tha(-1)yr(-1)), followed by cropland (10.03 tha(-1)yr(-1)) and shrubland (7.58 tha(-1)yr(-1)), while forest and grassland were <2.55 tha(-1)yr(-1). Slope and precipitation (contributing over 49.85% and 6.80% across ecosystems, respectively) were the dominant drivers of soil erosion, whereas vegetation covers consistently migrated erosion (-0.04 <= r <= -0.01). Human activity reduced vegetation cover (-0.15 <= r <= -0.08), thereby intensifying erosion. Overall, erosion intensity declined by 17.04% over the past three decades, yet management should prioritize bare land, cropland, and sensitive zones to strengthen restoration and prevent soil erosion.