Currently, as a result of the global climate change phenomenon, upstream flows as well as tidal currents are increasing rapidly the complexity, and this comes with an ever highest urbanization rate. Therefore, studying on downstream flood of reservoirs is essential, especially for medium and large reservoirs wherethere are high concentration of population at downstream. Featuring a vast region with low topography and influencing by tidal regime of the East Sea, the downstream of Long Song reservoir could be flooded heavily if there are serious flood. This paper will apply a mathematical-hydrological flow model combined witha one-dimensional hydraulic model of rivers and a two-dimensional modelof floodplains to calculate inundation levels at the downstream of Long Song reservoir, Binh Thuan province. Based on the results achieved from hydraulic calculations, several maximum inundation maps, corresponding to different discharge scenario in combination with extreme rainfall and tides occurring at the downstream, are established. The results of this study are a prerequisite to operate safely the reservoir and to minimize adverse effects for the downstream.