Abstract: With the hydrological data from Longkou Bay and adjacent waters, hydrodynamic patterns of current, wave, tidal prism and water exchange rate before and after the construction of artificial island groups are simulated using the numerical model Mike21. On this basis, the impact of the construction of artificial islands onto the hydrodynamic environment of Longkou Bay is discussed. The results show that the construction of artificial islands has significantly changed the characteristics of current field in Longkou Bay in addition to water movement path. Flow velocity within the bay is generally reduced due to the obstruction of artificial islands, and the local tidal current movement has changed from the reciprocating flow to a rotating flow. The flow direction also changed greatly, and the residual flow forms multiple vortices. As the local regional velocity increases, the residual flow velocity also increased outside the bay due to the rippling effect of dike heads, which result in the increase in tidal current. There is no obvious change in the form of flow movement. Due to the masking effect of the artificial islands, the significant wave height at the artificial islands and their surrounding areas generally decreases. In Longkou Bay, the maximum tidal range decreases in the north and increases in the south, and the change of tidal range is most obvious due to backwater. The construction of artificial islands has directly occupied the area of Longkou Bay, resulting in the decrease of tidal capacity, the increase of water exchange rate in the south and north, and the decrease of water channel in the north and in the area inside the artificial islands. The change of hydrodynamic environment caused by artificial islands is the main reason to the change in water exchange rate. The construction of artificial islands has weakened the hydrodynamic characteristics such as tidal current, wave, tidal capacity and water exchange of Longkou Bay, which is fundamental to the change of hydrodynamic conditions in the bay.