In order to ensure the safety of river channel flood and the bridges safe running, crossing bridge as the river management within the scope of construction projects must conduct flood impact assessment. Combining with the characteristics of bridge opening flow, the assessment contents,evaluation index and evaluation calculation method were systematically analyzed, so the assessment system of flood control impact assessment for crossing bridge was built. On this basis, taking practical engineering as an example, the paper made flood control comprehensive evaluation from the arrangement of bridge, bottom elevation analysis and analysis of influence on the river regime by calculation of flood control. The results showed that the assessment system is scientific and practical, it can provide reference for flood control impact assessment of other crossing bridge.