Apex model includes nine modules: meteorology, hydrology, water erosion, nutrient, soil temperature, crop growth, cultivation, crop environmental management and economy. The improved SCS Runoff Curve number method is used to estimate the daily runoff of the model. The original SCS model provides the Runoff Curve Number with a slope of about 5%. The modified Runoff Curve number can be used for other slopes. At the same time, the daily runoff curve number is adjusted according to the soil moisture content and its distribution. Must equation is used to simulate water erosion, which is improved by musle. The biggest difference between must and USLE is that runoff and peak velocity are used as erosivity indicators to simulate soil erosion and sediment yield. This not only improves the prediction accuracy, but also avoids the use of sediment transport rate, but also can be used for sediment yield estimation of word rainstorm.