Abstract: In this paper, by establishing a hydrodynamic model, a material transport model and an age model, a numerical simulation study was carried out on the hydrodynamic conditions and the process of pollutant transport and diffusion driven by the tides in the Liaodong Bay of the Bohai Sea. The results show that the runoffs have little effect on the flow field and water exchange process in the Liaodong Bay, and their impacts are mainly concentrated near estuaries. Driven by the tides, a complex circulation structure is formed in the Liaodong Bay. There are clockwise and counterclockwise circulations in the Liaodong Bay. A counterclockwise circulation is formed at the mouth of the Liaodong Bay, adversely affecting the transportation of materials from the bay to the open sea, and the materials in the top of the bay are exchanged with open sea mainly through diffusion process. The result of the age model shows that the pollutant from runoffs in the Liaodong Bay stays for a long time near the estuary, and takes a longer time to be transported to distant area. The pollutant entering the sea has adverse impacts on the water quality of local sea areas, especially on the water quality at the top of the Liaodong Bay.