The Government of Sri Lanka has been planning to build Colombo port city development project at the south of Colombo South port，where artificial water system,marine basin and artificial sandy beach have been built.The water exchange in this area is one of the most complex problems after building the project.Through the tidal physical model test,the hydrodynamic characteristics,flow capacity and water exchange of the water in the project studied.The result shows that big difference exists among each waters of the artificial river system.The hydrodynamic of southern marina basin is the weakest;the bend top position of the 3 corners in the Canal is stagnation regions and weak current recirculation regions both for south current and north current.But as long as small water level difference exists at the south and north entrances,a certain number of displacements flow may be formed.The model test result shows that water exchange capability is mainly depend on the unidirectional flow velocity and duration.Under experiment conditions the water exchange in the Canal can basically meet the requirement of fulfilling exchanging once per every 10 days and because the southern marina basin is concaved and has the weakest hydrodynamic,it cannot meet the water exchange requirement.