Southwest drainage tunnel of Hangzhou City intended to be arranged in the Jiuxi reach of the Qiantang River. In order to study the influence of embankment line optimization on flood discharge and tidal bore of Qiantang River, 2-D mathematical model was established to study height of tidal bore and the variation of velocity along the river during spring tide and flood of Qiantang River once-in-a-century after the implementation of the project. Results from numerical model calculation showed that when Qiantang River encountered a spring tide once-in-a-century, relatively obvious raised water level in front of area in north embankment could be seen, the maximum was up to 0.05 m, increase of water level on the south embankment was less than 0.01 m, and the velocity of flow in the south embankment increased by about 2% after the embankment line was optimized. However, the velocity of the water in front of the embankment in the upper and lower reaches had little change. When the Qiantang River encountered a flood once-in-a-century, water level in the local area of the engineering reach increased about 0.02m. Velocity in front of the embankment downstream adjacent to embankment increased by 1%~2%, velocity increased by 2 %~ 3% along the south embankment in the reach of the Puyan drainage and irrigation station. The influence on velocity in the front of the embankment of the Shanhusha Reservoir was relatively small. On the whole, the optimization of embankment line in Jiuxi reach had little influence on flood discharge and tidal bore in Qiantang River, had no effect on the water wading projects such as Metro Line 6, Zhijiang Bridge and Qiantang River Bridge in the area.