Long-term rainfall runoff plot experiment were carried out at slope citrus land of thin layer of purple soil in the three Gorges Region from 2002, and natural rainfall and surface runoff of each plots at each rainfall event were observed. Surface runoff of 37 rainfall events (2004—2007) of each plots were calculated by SCS method and MSCS method (modified SCS method), and MSCS calculation was proved more feasible for assessing surface runoff by the observation, SCS method, however, should be modified. The curve number calculated by rainfall runoff and SCS equation were greatly different from that listed in the handbook of USDA, but were quadratic correlated with rainfall volume. MSCS method, with the CN’ deduced by the quadratic model could calculate surface rainfall runoff at rainfall event with higher accuracy, especially when the rainfall volume surpasses 16 mm. MSCS, an empirical model originated from plot observation, is more feasible for surface runoff calculation compared with the original SCS method.