Runoff-sediment relationships are a key mechanism linking the hydrological cycle and geomorphological evolution, playing a fundamental role in watershed ecological security and water resource management. Recent soil and water conservation measures in the Loess Plateau region, including terraces, check dams, and re-vegetation, have profoundly altered watershed runoff and erosion processes, however, the response mechanisms of runoffsediment relationships to these conservation measures at the flood event scale remain unclear. This study uses a typical paired watershed in the hilly-gully region of the Loess Plateau (Jiuyuangou watershed, managed; Peijiamao watershed, unmanaged) as the study area. Based on observational data from 129 flood events from 1960 to 2020, methods such as multivariate statistical analysis and runoff-sediment hysteresis curves were employed to explore the evolution of runoff-sediment relationships and their driving mechanisms across three key periods: the baseline period (1960-1969), the engineering benefit period (1991-2000), and the engineering & reforestation benefit period (2011-2020). The results demonstrate that engineering measures significantly reduced the frequency of flood events, with the managed watershed showing a 56% reduction in flood frequency and a 92% decrease in sediment yield compared to the baseline period. Both watersheds exhibited significant decreases in the sediment yield to runoff ratio, and average suspended sediment concentration was markedly reduced. Vegetation restoration significantly reduced the sensitivity of sediment concentration to runoff drivers. In extreme rainfall events, soil and water conservation measures weakened the sediment supply process within the watersheds, driving the evolution of hysteresis loops from a simple anticlockwise pattern to eight-shaped or complex types. These findings provide valuable insights for optimizing soil and water conservation measures and ecological restoration on the Loess Plateau.