Soil organic C (SOC) loss is usually estimated in relation to sediment loss in the long term and at a large scale. Further studies on building a simple SOC prediction model for individual rainfall events are thus needed. Simulated rainfall experiments were conducted under two rainfall intensities (90 and 120 mm h−1) and four typical slope gradients (10, 15, 20, and 25°). Two types of soils (Suide and Changwu) with different textures were selected. Results showed that total sediment loss and SOC loss are correlated and that this correlation is particularly significant under lower organic C content (P < 0.01). The SOC enrichment ratio was high under 90 mm h−1 rainfall intensity and increased further by a low slope under interrill erosion. The relationship between average flow velocity and average sediment concentration for the Suide soil could be regressed with a linear function (R2 > 0.94), and the relationship between slope and runoff could be modeled with a quadratic function (R2 > 0.94). For the Changwu soil, both sediment concentration and runoff showed significant relationships with slope (P < 0.05). Soil organic C loss can be predicted simply by flow velocity, slope, SOC content, and SOC enrichment ratio. The prediction accuracy of the model was improved when the rainfall intensity was low. The results of this study can serve as a reference in enhancing the accuracy of SOC loss prediction.