【Objective】 This study aimed to investigate the erosion characteristics and sediment carbon loss patterns at different slope positions in the long and gentle slope farmland of the black soil region in Northeast China. The research focused on the full - growth - period coverage of soybean and maize, and at the same time, optimized the cover management factor (C - factor) model to provide a theoretical basis for erosion control and carbon sequestration in this specific geographic area. 【Method】 The research was conducted in the Fuhe small watershed of Bin County, Heilongjiang Province. Runoff plots were established at three distinct slope positions: the upper slope, middle slope, and lower slope. Through in situ artificial simulated rainfall experiments, the study systematically analyzed the impacts of soybean and maize coverage on runoff generation, sediment yield, and sediment carbon loss. The dynamic relationship between water and sediment was explored. Furthermore, by integrating two key vegetation parameters, crop coverage and plant height, a comprehensive and optimized C - factor model was constructed to improve prediction accuracy. 【Result】 The results demonstrated that under the coverage of soybean and maize, the runoff and sediment yield in all plots followed a "V"-shaped trend, initially decreasing and then increasing as the crop growth periods progressed. The optimal stages for reducing runoff and sediment were identified as the beginning of the grain filling stage for soybean (with reduction rates of 33.76%~40.33% and 89.96%~94.54%, respectively) and the tasseling stage for maize (46.32%~54.23% and 92.67%~96.29%, respectively). Comparatively, maize exhibited superior erosion resistance capabilities to soybean. Regarding spatial distribution, the erosion sensitivity across different slope positions followed the hierarchy of lower slope > middle slope > upper slope. Statistical analysis showed that runoff volume, sediment yield, and sediment concentration in runoff were all highly significantly negatively correlated with both crop coverage and plant height. The optimized model integrating these parameters (C = -0.595log(0.01V)×(0.00491H+1.65048)) demonstrated a significant improvement, reducing the root mean square error (RMSE) by 59% compared to traditional models. Additionally, sediment carbon loss was significantly negatively correlated with crop coverage and plant height. Maximum crop coverage was found to reduce total sediment carbon loss by 91.04%~96.12%, with the most effective stages being soybean grain filling and maize tasseling. Notably, 32.65%~69.91% of the total sediment carbon loss was contributed by large sediment particles (> 0.25 mm). 【Conclusion】 This study successfully reveals the specific patterns of soil and sediment carbon loss under the full-growth-period coverage of large-grain crops on long and gentle slopes in the black soil region. The research highlights the critical role of crop phenology and slope position in influencing erosion processes. The findings and the optimized C-factor model provide a robust theoretical foundation for developing effective erosion prevention measures and enhancing carbon sequestration strategies in the long and gentle slope farmland of the Northeast China black soil region.