To evaluate the marine environmental impacts of coastal reclamation, a numerical model of the proposed Kau Yi Chau Artificial Islands reclamation project in the South China Sea was developed using the ECOMSED model. The model was employed to systematically investigate the hydrodynamic changes induced by the reclamation project and their subsequent effects on suspended sediment dispersion and pollutant transport before and after project implementation. The results show that: (1) The reclamation project significantly alters the local tidal tidal current structure, with maximum flow velocities at flood and ebb peaks increasing by approximately 97.9% and 157.3%, respectively, in adjacent areas, accompanied by maximum flow direction deviations of 32.7° and 49.2°. However, these impacts exhibited spatial attenuation, becoming negligible beyond 7.5 km from the project site. (2) Compared with the conventional construction method, the synchronous construction method can significantly reduce the dispersion range of suspended sediment, decreasing the area exceeding Class I and II seawater quality standards by 7.7%. (3) The pollutant diffusion simulation under the synchronous construction mode indicated that the affected area in which petroleum pollutants exceeded the Class I and II seawater quality standards reached 1.36 km2, while the affected area in which anionic surfactants exceeded the Class I seawater quality standard reached 18.98 km2. In addition, heavy metals such as Pb and Cu also exhibited a certain degree of spatial diffusion. The results of this study can provide scientific data support for the ecological and environmental impact assessment of coastal reclamation projects.