In order to study the spatial and temporal dynamic changes of water salinity at a large scale in coastal reservoirs, the source and sink terms in the salinity transport equations of the hydrodynamics and water environment numerical simulation model, i.e., the Delft3D model, were improved, and the improved Delft3D model was validated by comparing the simulation values with theoretical values. The results show that the improved Delft3D model can be applied to the simulation of water salinity change in a proposed coastal reservoir, and quantitatively predict the salinity change processes of the reservoir caused by the sediment salt release under different scenarios.