Abstract: This study was carried out on the case of the East Coast New City of Shantou, to analyze and explore the drainage system and water environment characteristics of coastal urban areas. By long-term observation records, continuous hydraulic and water quality online monitoring, as well as regular on-site water quality sampling and analysis, the study revealed that within the research area, the water levels in the rainwater pipes were influenced by the reverse flow of tidal water levels, displaying a consistent pattern with the tidal cycle. It was found that anti-reflux facilities were unable to completely prevent tidal water from permeating the drainage pipe network through shallow underground infiltration. Consequently, high-salinity external water brought in by tides easily entered the sewage pipe system due to backflow caused by the mixing of rainwater and sewage, or pipe leakage. Furthermore, rainfall-induced non-point source pollution was channeled into the receiving water body, which demonstrated a certain capacity for assimilation. The number of days with excellent water environment accounted for 86.6% of the whole year. However, inferior water environment day primarily occurred during periods with lower rainfall, accounting 3.8% of the whole year. In spring, autumn, and winter, diatom species, such as Skeletonema and Navicula formed algal blooms in the inner river channels, directly impacting the appearance condition of the water environment. The findings of this study can serve as a reference for the design and optimization of drainage systems in coastal urban areas, as well as the development of strategies related to urban flooding, pollution control, pipeline maintenance and operation, and water environment management.