Abstract: A series of dredging,hydrologic and topographic in situ measured data are collected to study interannual variations and associated causes of siltation in the 12.5 m Deepwater Navigation Channel (DNC). The results show that:the annual siltation intensity of DNC is characterized by a steady decrease. The siltation intensity of the South Channel to the Yuanyuansha Channel in the upper reach of DNC obviously decreased. The siltation intensity of the North Passage was fluctuating from 2011 to 2015,but it decreased in 2016—2017. The decline of siltation in the South Channel to the Yuanyuansha Channel is mainly related to the decrease of bed load transported from the upper reach and the reduction of elevation difference between the nearby shoals and troughes. The project of the Sand-retaining Dike and the deline of the impact induced by typhoons are the main cause of the decrease of siltation in the North Passage in 2016 and 2017.The large runoff in 2016 & 2017 led to the turbidity maximum zone and the position of main siltation in the North Passage moves downward. The effect of fluvial sediment decline on the siltation of DNC has been emerged gradually. In future,due to those influences,the siltation of the South Channel to the Yuanyuansha Channel will experience declining furtherly and maintain a lower value,while the siltation in the North Passage also will keep a steady downward trend.