Abstract: A lagoon-type tidal inlet system with a larger outer delta, namely Laolonggou, is located in the Caofeidian sea area of China's Bohai Bay. With the decrease of the sediment supply in last decades, the seabed was eroded obviously on the sea-side area of the Dongkengtuo barrier island, and the isobathic lines retreated toward the coast. The deep channel of the West branch was also in an erosion state, and the change of erosion-deposition in the outer delta was mainly affected by the Dongkengtuo island. The obvious erosion in the eastern channel is mainly caused by the extension of the Dongkengtuo head land, leading to the increase of the current velocity. The western channel narrows and moves toward the west, due to the extruding of the eastern channel. In recent years, the eastern channel development has been slowing done together with decreasing in the western channel swing amplitude, which is also due to insufficient sediment supply.