The encircled basin can break waves and reduce sands, but it is disadvantageous for the water exchange flux for the harbor. Based on the project of Xuwei harbor in Lianyungang, the semi-exchange period of the water body is simulated and calculated with a convection-diffusion model under the wave-current interaction.The improving measures and their effect for the water quality are also investigated. Results indicate that the hydrodynamic condition is the key factor influencing the semi-exchange period of the water body, including the effect of tidal type and the position of harbor basin. The effect of measures depends on the improvement degree of the hydrodynamic condition in the encircled harbor and the size of the water exchange capacity.