Abstract: Based on the measured hydyographic data, analyzed results of sampled sediment, and the bottom elevation data in the Jiantiao Harbor, the dynamic sedimentation processes and the balance mechanism of erosion and deposition are discussed. It is pointed out that the sediments are eroded during the spring tide and deposited during the neap tide, the bottom of the harbor is eroded and the tidal flat is deposited under the action of the tidal currents. With the change of the erosion and deposition in the space, the slope of the harbor coast is steeped and finally becomes, unbalanced, the slope of the coast slides and collapses, which results in the new balance botween the tidal flat and the harbor bottom.