There are three main reasons for the formation of tidal waves in Qiantang River. First, the mouth of Qiantang River is trumpet-shaped at the entrance of Hangzhou Bay, and gradually becomes shallower and narrower inward. The tide flows backward, and the narrow river surface in Neijiang limits the spread of tidal waves, forming a magnificent tidal bore. Every year from August 16th to 18th of the lunar calendar, the spring tide is the highest. Secondly, there is a huge sandbar in the estuary. When the tide rushes in and meets with strong resistance, it will definitely lift the sky. The front wave was suppressed and the back wave rose again. Waves push waves, and waves overlap. The tide naturally surges and growls, coming like an avalanche. The height of tidal head can reach more than 3.5m, and the tidal range can reach 10m. Third, the spring tide is related to the gravity of the moon and the sun. Tide is a periodic fluctuation phenomenon of seawater under the gravity of the moon and the sun. The tide in Qiantang River is mainly caused by ebb tide, so it is also related to the gravity of the moon and the sun.