Abstract: Background Extreme rainfall events are becoming more frequent under climate change, and are substantially altering hydrological and erosion processes in agricultural systems. Contour ridging is an effectively agricultural practice applied worldwide on sloping cropland. Under extreme rainfall, however, rapid water accumulation in furrows can produce a distinct rainfall partitioning regime among ponding, infiltration, and runoff. More importantly, such conditions can drive the evolution of erosion processes from interrill to headward and rill stages, thereby increasing the risk of soil erosion on contour-ridged slopes. Nevertheless, the dynamic mechanisms of rainfall partitioning, runoff generation, and sediment yielding changes across erosion evolution processes remain insufficiently understood. To address the knowledge gap, this study elucidated the rainfall partitioning pattern and the dynamic processes of runoff and sediment yielding during the erosion evolution processes within contour ridge system under extreme rainfall, which can improve our understanding of erosion processes and mechanisms in contour ridge systems, and provide a scientific basis for evaluating and optimizing soil and water conservation measures on sloping croplands under extreme rainfall conditions. Methods Simulated rainfall experiments were conducted with an intensity of 100 mm/h for 70 min duration under three ridge grades (2°, 6°, and 10°) and three ridge widths (50 cm, 60 cm, and 70 cm). The dynamic evolution of ponding, infiltration, and runoff, as well as the variations in runoff and sediment yielding processes, were analyzed during the erosion evolution processes of interrill, headward, and rill erosion in contour ridge system under extreme rainfall. Results Under extreme rainfall, erosion evolution on contour-ridged slopes was dominated by headward erosion, while rill erosion still occurred on 44% of slopes. During this erosion evolution processes, rainfall was primarily partitioned into infiltration (57.11%~70.25%), followed by runoff (6.39%~50.08%). As erosion evolved from interrill to headward and rill erosion, the rainfall partitioning pattern shifted from infiltration-dominated to runoff-enhanced. Runoff rate showed an increasing trend with rainfall duration during interrill and headward erosion, whereas its variation during rill erosion depended on the degree of rill development. Rill erosion contributed 44.17%~94.23% of total runoff, with runoff averaging 20.66 times and 4.65 times higher than that during interrill and headward erosion, respectively. The development of headward and rill erosion significantly intensified erosion intensity. During the interrill-headward evolution processes, headward erosion contributed 25.42%~90.41% of the total sediment yield. During the interrill-headward-rill evolution processes, rill erosion accounted for 65.8%~96.12% of the total sediment yield. The relationship between sediment yield and runoff varied with erosion evolution. During the interrill-headward evolution processes, sediment yield and runoff in interrill and headward erosion followed exponential and linear relationships, respectively. For the interrill-headward-rill evolution processes, sediment yield and runoff showed quadratic relationships in both interrill and headward erosion, whereas this relationship shifted to a power function after rill development. Conclusion The evolution of erosion processes within contour ridge system under extreme rainfall significantly altered rainfall partitioning pattern. Rill development was the dominant process driving increases in both runoff generation and soil erosion. Meanwhile, the evolution of erosion processes also changed the runoff-sediment relationship.