Abstract: According to the depth sounding data of 1992, 1997 and 2006, an comparative topographical analysis was conducted along the transection at the mouth of the Laolonggou deep trough and the longitudinal section roughly along the axis of the trough. Results show that the deep trough tends to be silted up with the water getting shallower. In general, the thickness of the silted deposits is between 0.61m to 4.8 m, and the axis of the trough shows a tendency to wander from east to west. Since 2004, once the reclaimed road leading-to-island cut off the shoal tidal channel, the flow and the velocity of the tidal current have remained to decline, and this is the main reason to make the Laolonggou deep trough shallower. The siltation probably happened mainly at the initial stage during 2004 to 2006 right after the road blocked the shoal tidal channel. This research result also proved our previous hypothesis that blocking the shoal tidal channel would initiate siltation in the Laolonggou deep trough.