Abstract: The damage to bridges can be caused by scour around piers， so a more accurate calculation of pier scour depth is of great significance. In order to evaluate the scour depth accurately， unstructured grids are used and large and small models are nested to built a two-dimensional hydrodynamic numerical model. This model is calibrated using the measured tidal levels， current speed and current direction data. On this basis， the tidal power in the sea area is also studied， and the influence around the bridge project is analyzed， and the local scour of many cross-sea bridge piers is calculated by introducing some reliable formulas， which are the pier local scour formula 65-2 and the modified 65-1， the existing norms recommended in China， and the formula HEC-18， recommended in USA industry standards. The results show that the construction of bridges has a little impact on the tidal current in the bridge area and the the largest possible local scour depth occurrs in the main pier of the main bridge.