The erosion depth of river-crossing tunnel site section is directly related to the economics of project design and the safety of project operation,so reasonable prediction is of crucial importance.On the basis of studies of domestic and foreign scholars,we take the Yangtze River tunnel in Chongqing as a case study,analyze the riverbed evolution and the erosion changes of tunnel section according to the data of hydrologic conditions and the reservoir operation in the Three Gorges fluctuating back water area,and predict the erosion depth of the riverbed at river-crossing tunnel section under adverse hydrological conditions by some commonly used erosion calculation formulas.The results show that the predicted erosion depth of river-crossing tunnel site section is 4.8 m.The achievement can provide the references for the similar projects.