The main objective of this research is to simulate and study the effects on inundation and environment in the downstream of Lung-luan-tan reservoir due to dam break. This study consists of 3 major works. Firstly , simulating the outflow hydrograph for trapezoidal rectangular cross-section dam break shapes by using DAMBRK model under the scenario of that reservoir is full and 25Yr,50Yr or 100Yr return period storm occurred simultaneously. Secondly, the outflow hydrograph is used as the initial condition for SOBEK model for estimating the flooding condition in time and space after the dam break. Finally the simplify water quality model was applied for simulating water quality and to analyze water quality and ecosystem impact in Heng Chuan area.