Abstract: Human activities have led to a substantial accumulation of nitrogen in urban ecosystems, exacerbating nitrogen pollution in coastal waters. Addressing this pollution poses significant challenges. Shandong's coastal cities, as pivotal drivers of national development and gateways to the world, were selected as the focus of this research. Utilizing material flow analysis, we constructed a model to assess nitrogen load in the aquatic environment of these cities, quantifying nitrogen input characteristics and their contributions. Nitrogen metabolism efficiency indicators were employed to uncover potential nitrogen reduction capacities and variations among the cities. Subsequently, the primary factors influencing nitrogen inputs to the water environment in each city were identified. The findings revealed that: (1) From 2002 to 2021, the nitrogen input intensity in the water environment in Weihai, Weifang and Binzhou exceeded the average level of 35 kg/hm² for Shandong´s coastal cities. The variation trends in the intensity of nitrogen input across cities, relative to their respective averages, exhibit distinct characteristics, highlighting the instability in nitrogen pollution control. (2) In terms of emission structure, crop cultivation, livestock and poultry breeding, as well as aquaculture, are the primary sources of nitrogen emission. Household consumption and waste treatment subsystems also contribute significantly. Meanwhile, nitrogen input from industrial manufacturing was controlled, while the atmospheric nitrogen deposition exhibited an increasing trend. (3) The nitrogen environmental loss rates for livestock and poultry breeding subsystems were high, indicating substantial potential for emission reduction. Nitrogen input into the water environments of different cities is influenced by a variety of factors. Nonetheless, a consistently positive correlation exists between nitrogen input and the rate of nitrogen application per unit of agricultural land across these cities. This research underscores the distinct spatial and temporal distribution characteristics of nitrogen input in the water environment of Shandong´s coastal cities. China′s targeted management strategies have been instrumental in fostering positive advancements in controlling nitrogen pollution in these regions. Each city should develop effective strategies for key subsystems to achieve nitrogen emission reduction and enhance the quality of the aquatic environment.