Dam's weir is one of the most important dam structures that play a significant role in flood routing in the dam reservoir. Since the dam's level significantly affects the storage, changing the level and increasing its height can increase storage volume and flood control in some situations. Shahid Abbaspour Dam (Karoon 1), due to the reduction of the dam volume, affected by the accumulation of sediments and the construction of upstream reservoir dams, it is necessary to increase the dam's weir height. This research used a numerical simulation of the weir with Flow3D software. Moreover finally, after the model verification, its height was increased. All model outputs were compared with the physical model results and the dam's data, and the simulation fit well with both systems. According to the model results, in exchange for changing the level of the dam from 510 to 513.5 and the fully opening of the Gates, no negative pressure is created on the weir surface, and the cavitation index is within the allowable range.