The melting of glaciers as a result of continued global warming is raising sea levels and further affecting groundwater levels and soil water and salt conditions in coastal wetlands. In recent years, studies on ecological stoichiometry have increased rapidly. Relationships between ecological stoichiometry indicators of plants and soils have been widely investigated. Understanding the ecological response strategies of plants to soil water and salt environment in coastal wetland could provide important theoretical basis for the protection and restoration of coastal wetland vegetation. Therefore, the dominant plant Phragmites australis in coastal wetland was selected as the research object. By investigating the leaf C/N/P stoichiometric characteristics of Phragmites australias and soil under different groundwater levels in the Yellow River Delta, this study analyzed the coupling relationship between the stoichiometric characteristics of Phragmites australias and soil environmental factors. The results showed that: (1) During the growing season, both the leaf N content and leaf N/P of Phragmites australias showed a decreasing trend. In the middle of the growing season, the leaf N/P ratio of Phragmites australias was between 14 and 16, indicating that the growth of Phragmites australias in this area was jointly limited by N and P. The soil C, N, P content and N/P in the Phragmites australias community all showed a decrease trend from the surface to the bottom of soil profile. The soil C and N content in the Phragmites australias community was significantly lower than the average soil C and N content within China. However, the soil P content is relatively close to the average soil P content within China, indicating that the soil N content in this region is relatively lacking, while the P content is relatively abundant. Correlation analysis showed that there was a highly significant positive correlation between leaf C content and soil N content (P < 0.01), and a significant negative correlation between leaf N content and groundwater level (P < 0.05). There was a significant positive correlation between leaf P content and soil conductivity (P < 0.05). There were significant or extremely significant correlations between leaf P content and soil element content (C, N or P), and there were also significant correlations between leaf element ratio (C/P or N/P) and soil element content (C, N or P). Redundancy analysis showed that the explanatory power of environmental factors for the first axis could reach 73.22%. Correlation analysis combined with redundancy analysis indicated that soil P content, soil N content, soil conductivity, and groundwater level were important driving factors affecting the ecological stoichiometric characteristics of Phragmites australias.