Abstract: A 2D numerical model is used to simulate flood level in the demonstration of an approach channel excavation scheme and its sedimentation after excavation of the Sanyanggang floodgate. Research results show that the channel having 120 m in width and -3.5 m in elevation depth can not fit the flow of 6 400 m3/s. Beach roughness sensitivity is analyzed， and several excavation schemes of the downstream approach channel with different lengths and elevations are calculated in this paper. According to the results above， a proper excavation scheme is: width equal to lock chamber， length of 2.2 km， the left beach elevation of -1.0 m， and the right of 0 m. Amounts of sediment in the downstream channel in three typical years of being dry, normal and wet are 181,000 cubic meters， 121,000 cubic meters and 36,000 cubic meters. If three continuous years are dry， the siltation volume is about 500,000 cubic meters. In this condition， water level will rise by 0.12 m when encountering flood discharge of 6 400 m3/s. In this moment， sediment dredging is necessary.