The study of design lowest navigable stage in water and sediment variation environment is an important issue in waterway regulation engineering.Based on analyzing the riverbed scour and silting evolution in the key reaches of Xijiang River from Jieshou to Zhaoqing,and according to the hydrological regimes characteristics,we use the synthetic duration curve method to calculate the minimum design navigable discharges.Then we obtain the influence magnitude and range of tide,unsteady flow,waterway regulation engineering on design lowest navigable stage by means of flow mathematical model.Finally we get the design lowest navigable stage along the river.The study is a good reference for the calculation of design lowest navigable stage in tidal reaches below the dam in water and sediment variation environment.