Bridges are high-cost and high-tech infrastructure which are essential to public transportation. However, it is subject to various environmental impacts. Especially, as it is built in the river, a detailed hydraulic study is essential before the construction of the bridge as it may cause a huge impact to the existing river conditions, livelihood near the river bank and river navigation. This study aims to quantify hydraulic responses at Prek Tamak bridge when another bridge is parallelly built next to it. To reach its aim, a 2D Mekong Hydraulic model at that area is constructed by using HEC-RAS model. Observed flood level played an important role in model calibration. Consequently, hydraulic analysis was developed to observe hydraulic condition near the Prek Tamak bridge before and after another bridge is built by considering on peak discharge observed between 2013 and 2017. The variation of velocity, shear stress and inundation area are assessed in this research. As a result, the flow’s velocity and shear stress increase along the centerline and right bank of the river while decreasing near the left bank. In addition, inundation boundary extension ranged from 15m to 60m are spotted at upstream of the bridge. A detail slope stability analysis along the river bank need to be carried out prior to the construction.