Abstract: The rainfall,water-level and outflow hydrograph corresponding to the over-dam inflow flood are analyzed.The inflow flood recurrence interval is evaluated,and the coordinate of outflow peak at the falling segment of the inflow hydrograph is found.The irrationality using the water-balance to reproduce inflow flood hydrograph is discussed.A simple triangle method for reproducing inflow flood of the reservoir is put forward,and the steps and conditions are proposed and discussed.The results and analytical method may be helpful to the operation and design of mountainous reservoirs.