Abstract: The water-blocking ratio is an important parameter in the flood control evaluation of river-crossing bridges.However, the definition of the water-blocking ratio in the existing codes only considers the water resistance area, but ignore influence of the velocity distribution on the water-blocking ratio, resulting in a large difference in the section velocity distribution calculated when main channels and the side beaches of rivers are developed.According to the square relationship between the local head loss caused by bridge water-blocking and the water flow velocity, we use the square of a ratio of water flow velocity at piers before bridge construction to the average flow velocity of the whole section at the bridge site as a weight coefficient to the flow velocity.Then this weight coefficient is used to correct the water-blocking area of piers, so as to improve the calculation formula of water-blocking ratio and it is applied to an example to verify the rationality.The calculation results show that:(1) The unit water-blocking areas of piers located in the main channel had a great influence on the water-blocking ratio, while the unit water-blocking areas of the piers located in the side beach had little influence, and the greater the difference in flow velocity, the greater the differences in the degree of water blocking effects;(2) When the ratio of main channel flow velocity to the average flow velocity of the whole section reached more than 1.17,the influence of the weight coefficient of flow velocity on the water-blocking ratio should be considered.The water-blocking ratio based on the flow velocity weight coefficient can more truly reflect the water-blocking effect caused by the difference of flow velocity, which is closer to the actual water-blocking situations of projects.In order to effectively reduce adverse impacts of piers on flood control, long-span structures are suggested to cross the main channels of rivers.