To counter the phenomenon that the total siltation in 250 000 DWT approach channel at Lianyungang area is less than 150 000 DWT，we probe into the cause for it based on the analysis of the actual measured data about channel siltation and dynamic environment of water and sediment before and after engineering by the flow mathematical model.The result indicates that both dredging engineering and encircle breakwater engineering will have impacts on silting of the approach channel，but the breakwater is more essential.Dredging engineering mainly makes the flow rate of the channel slow down and hence it slightly increases the channel siltation;encircle breakwater engineering makes the flow rate near the entrance increase significantly so that the approach channel siltation reduces sharply.In addition，the protective effect of the encircle breakwater prevents the sediment from entering the port and then the siltation of inner fairway section also decreases in a large scale.