The middle reaches of Wei River are wide and main channel narrow deep which flood control situation is grim. Through the example of the proposed construction of the Xi 'an to Famen Temple Inter-city Railway Bridge, the flood control evaluation content, backwater analysis of different problems, the physical model test, numerical simulation and simplified experience formula method, respectively, are discussed in this paper. In addition, simplified formula method was selected to calculate the backwater. The railway and highway hydrological specifications included in the formula was chosen after four empirical formulas are compared and proved. The height of backwater of this bridge due to flood once in a hundred years can be calculated by Cao Ruizhang formula recommended by “Code for hydrological survey and design of highway engineering” after consulting data and analyzes the actual situation of the bridge site, and the height is 0.27 m. The safety of the project is confirmed by the calculation of the backwater of the continuous bridge and the check of the bottom height of the bridge. This study provides a reliable method and basis for the calculation of backwater of Bridges in safe flood passage in the middle reaches of Wei River.