Abstract: The norm of hydraulic design of railway bridge in China does not consider the effect of the deflect degree of the flow direction under bridge. By extending the calssical fluid mechanism result about the flow field round a pier to two piers,the formula of the deflection degree of flow direcrion is deduced with the assumption that the field around two piers simulates that of one pier. The formula shows that the deflect degree first increases,then decreases,and its maximum increases while the compress rate decreases. The formula fits with the laboratory data well in some condition,and corrects the mistake that the deflect degree only increases while the bevel increases.