The growth history of the Northeastern Tibetan Plateau (NETP) is enigmatic, with debates on when and how the NETP significantly uplifted. Here, we use a numerical landscape evolution model to quantitatively investigate the ~20 Ma growth history of the NETP by studying the formation history of the upstream Yellow River (UYR). Compared to the observed river profiles, erosion rates, the trend of acceleration time of deformation, and paleo-elevation, our modeling results suggest that the long-term growth history of the NETP consists of an early block uplift (~20-12 Ma) and a late outward propagation uplift (~12-0 Ma). Before ~12 Ma (middle Miocene), the NETP was uplifted via a block growth, with major uplift in the south part. Subsequently, the high (~5 km) NETP has been uplifted via a northward propagation growth until the present-day time. We further suggest that pure headward erosion unlikely formed the observed river profile of the UYR over the past few million years. Our modeling thus reconciles the long-term outward growth of the NETP and the UYR profile, suggesting a downstream migration of high erosion rates, which is fundamentally different from the headward erosion of small mountain rivers. The downstream propagation of fluvial erosion may commonly occur in the outward-growing plateau on Earth.