Rainwater regulation, which aims to harness surface runoff (Q s) and recharge soil moisture (SM) to support vegetation growth, plays an increasingly vital role in sustaining ecosystem functions in water-limited regions. However, the current understanding of how rainwater regulation modifies land surface processes and influences regional ecohydrological dynamics remains fragmented. To bridge this gap, this study incorporates rainwater regulation into the Noah-MP land surface model to systematically evaluate its effects across China's Loess Plateau from 2001 to 2018. The results show that rainwater regulation elevates evapotranspiration (ET) and subsurface runoff (Q sub), both rising in concert with precipitation (P) over the Loess Plateau. When Q s is fully utilized, approximately 52% is allocated to ET, 21% to Q sub, and 27% to water storage (S). Additionally, the leaf area index and water use efficiency (WUE) exhibit marked annual increases of 6% and 3.7%. Notably, SM in the northwest Loess Plateau declined significantly after rainwater regulation implementation, which can be attributed to redistribution driven by soil hydraulic properties and enhanced vegetation growth. These ecohydrological changes display distinct spatiotemporal variability and a clear cumulative effect over time. Importantly, our findings indicate that even under stationary regional P, optimizing rainwater regulation intensity can further improve vegetation growth outcomes. By integrating rainwater regulation into a land surface modeling framework, this study offers a systematic perspective on the underlying ecohydrological mechanisms of large-scale rainwater harvesting and provides a scientific basis for designing ecologically sustainable and economically viable rainwater regulation strategies.