Revealing how vegetation growth affects the rainfall-runoff erosion process of a basin is a key research focus in ecological engineering. Most existing studies have explored this topic from the perspectives of vegetation coverage and type changes, while few have investigated the impact from the viewpoint of the vegetation growth process. This study aims to elucidate the functional mechanisms linking vegetation growth with rainfall-runoff erosion and develop similarity criteria and simulation methods. To achieve this, we constructed threedimensional models of rainfall, soil, and vegetation in a small watershed to explore the relationships between vegetation growth and the watershed's near-surface hydro-meteorological processes. We identified the functional mechanisms between vegetation growth and rainfall-runoff erosion, and proposed a similarity criterion (lambda PHY=lambda rho lambda l) along with corresponding simulation methods. Results showed that for small watersheds in the Loess Hilly-Gully Region under normal annual rainfall, the thresholds for vegetation restoration are as follows: vegetation coverage <= 75%, vegetation height <= 10 m, biomass <= 175 t/hm2, and the water resource carrying capacity for vegetation <= 583 g C/m2. This study provides a novel approach for investigating watershed nearsurface meteorological-hydrological processes and ecological benefits.