Flood investigation is the basis of flood prevention and control，that provides important data support for the mountain flood disaster prevention and control project construction.Based on the field investigation of the flood area in the Tongbu River of Nancheng county，we take the Liantang village of Longhu town as the research object.Through the data of the measured river section，we obtained the relationship between water level and flow，then we can determine the peak flow of the floodmark position.The peak flow at different frequencies is obtained by the instantaneous unit hydrograph method，and then we can obtain the flood return period of the research object.It will help to grasp the historical flood magnitude and flood risk，and provide basic data for warning index and risk assessment of mountain flood disaster.It is of reference significance to the analysis and determination of the flood recurrence period in similar mountain flood areas.