Abstract: The Laolonggou deep trough was mainly depositional in the early stage after the shoal tidal channel was blocked by the Caofeidain reclamation project. It was deepened in the middle stage because of large-scale dredging for land reclamation and partially sands mining along the trough. The comparison of the depth sounding data of the three profiles crossing the Laolonggou deep trough between October 2008 and October 2011 suggests that the Laolonggou deep trough has returned to depositional after the Caofeidian reclamation project blocked the shoal tidal channel in 2009, since the project had come to an end, and the activities to take sediments from the seafloor for reclamation has decreased gradually or even stopped, when the shoal tidal channel has not been opened yet. It further proves our deduction that blocking the shoal tidal channel would cause deposition in the Laolonggou deep trough. Therefore, it is necessary to open up the shoal tidal channel again, and try to recover the current system to some extent, so as to alleviate the deposition in Laolonggou deep trough and protect the marine environment from siltation.