The flood peak level ofmountain river changes abruptly and the lasts short.Since the forbidden level stipulated by the maritime sector is generally lower than the national standard,we can decrease the maximum navigable water level of navigation structures to save construction investment.Thus we need to explore the appropriate standard.Analyzing the hydrologic data of 7 hydrometric stations in Sichuan province and 4 hydrometric stations in Chongqing city,we draw the conclusion that it is difficult to control the stage drop adopting the guarantee rate method instead of the frequency method directly,and it is feasible to lower the standard of the flood return period appropriately.