Delft3D is a powerful software package developed by WL Delft hydraulics of Delft University in the Netherlands, which is mainly used in free surface water environment. The software has a flexible framework, which can simulate two-dimensional and three-dimensional flow, wave, water quality, ecology, sediment transport and bed topography, as well as the interaction among various processes. It is one of the most advanced hydrodynamic water quality models in the world. Delft3D system is widely used in the world, such as the Netherlands, Poland, Germany, Australia, the United States and so on, especially the United States has a long history of application. The Delft3D system has been used in Hong Kong since the mid-1970s and has become a standard product of the Hong Kong Environment Agency. Delft3D has been applied more and more in mainland China since the mid 80s, such as Changjiang Estuary, Hangzhou Bay, Bohai Bay, Taihu and Dianchi. Delft3D is the most advanced and complete 3D hydrodynamic water quality model system in the world, especially the unique support surface format.