In these days, drought and flood occurs more frequently throughout the world due to an unusual exchange in the weather. As the localized torrential rainfall increases, the possibility of flood occurrence such as river flood, bank decay, reservoir destruction also increased due to worn out or impairment of the hydraulic structures caused by flood damage. Especially, there have been several historic localized torrential rainfall in Chullabuk-do and Chullanam-do, there is no enough research done about the risk of the A dam decay after its construction nor the flood-risk mapping of main A river. Therefore, in this research, postulated dam failure of A dam during maximum possible precipitation is proposed and investigated in order to calculate not only the peak flood for the continuous collapse time, but also the flooding hour, flooding type and flooded area extent that hourly influences B county, C county by using the MIKE FLOOD model. In analyzing the receptive capacity of the embankment in the main stream of the A River, except the mountainous region in its upstream, the receptive capacity of the embankment is higher than the planned flooding level and the existing embankment height, the embankment water level in the B region occurred up 8.80m in maximum, the embankment water level in the C region occurred up 2.27m in maximum. By using the flooding occurrence point, the maximum flooding depth, the flood graph of time and season, of the B and the C region to be caused by the breaking of the A river, the forecast and alarm and disaster prevention for the flood of the A river basin in the future should be planned.