A 2D tidal current and sediment numerical model was established to simulate the flow field and the sediment concentration during the survey of disposing spoil in Lianyungang waterway. Based on the good certification, the law of the diffusion of the suspended sediment from dredged spoil disposing and its influence on the sediment concentration of nearby water area were studied in different cases. The result of the study showed that if two drag suction dredgers with 4 500 m3 tank volume disposed the dredged spoil at the mud dumping area simultaneously when the flow direction was bias to the nearby waterway, the suspended sediment would spread quickly along the flow direction, the high-concentration water would not last for long and covered a short distance, also it had little influence on the sediment concentration of nearby waterway.