On the Vu Gia Thu Bon basin, in recent years (2009, 2011 and 2013) the problem of reservoir flooding discharge is always a matter of debate, although the operation in flooding discharge of these lakes met the standard process, but the way of fulfilling reservoirs and discharge rapidly after that were unsafe for the downstream. In this study, the authors applied the HEC-RESSIM model for simulating the reservoir systems in the Vu Gia - Thu Bon basin, hence providing the proposed reservoir operating rules in the case of the water level (before flood) smaller than flood prevention water level for safe discharging safely for downstream as well as not affecting the power generation target of these reservoir.