Abstract: The Lake Illawarra, as an almost landlocked coastal lagoon is rather famous in southwestern Australia. It is a fragile residual landscape of the Holocene transgression. The lagoon is a shallow elongated waterbody, with a water surface of 35 km² and a catchment of 270 km². The long-term sedimentation rate is less than 1mm/a, and the short-term sedimentation rate is about 5-6 mm/a. Influenced by land-sourced sand and heavy metal contamination, the lagoon has been in speedy degeneration in recent decades. Recently, various measures have been taken to stop the input of land-based sediments, deepen the lake, strengthen the lakeshore, stop lake breeding, widen tidal channels, and shorten the exchange time of seawater and lakewater. The water quality of the Illawarra lake is improving and some species of fishes are moving back. The lake has become an ideal place for tourists. The experience of remediation of the lake is helpful for coastal lagoon research.