Abstract: Phragmites australis is a widely distributed dominant wetland species with strong ecological adaptability. To elucidate the seasonal dynamics of salinity stress on the photosynthetic physiological adaptation mechanisms of P. australis, we selected low-salinity (salinity 1.1) and high-salinity (salinity 16.1) plots in the Yellow River Delta wetland and measured the dynamic changes in photosynthetic characteristics of P. australis during the growing season from June to September. The results showed that in the early growth stage (June), a high-salinity environment significantly inhibited the photosynthetic capacity and carbon assimilation efficiency of P. australis. However, during the vigorous growth phase (July−September), key parameters such as the maximum net photosynthetic rate and initial carboxylation efficiency were significantly higher in high-salinity plants than in low-salinity counterparts, and both light and CO₂ response curves shifted upward, indicating enhanced photosynthetic potential. Although the plant height and basal diameter of low-salinity P. australis were significantly higher (by 57.1%−219.3% and 36.2%−38.9%, respectively) than those of high-salinity P. australis, the relative chlorophyll content and plant density of high-salinity P. australis were significantly higher (by 13.3%−20.1% and 105.4%−300.0%, respectively) than those of low-salinity P. australis. Furthermore, during the early to mid-growing season, high-salinity P. australis maintained net photosynthetic rates comparable to those of low-salinity plants by enhancing water-use efficiency and the activity and content of Rubisco. Notably, high-salinity P. australis still maintained higher stomatal conductance and net photosynthetic rates at the end of the growing season, which may be related to its adaptive regulatory mechanism developed under prolonged salt stress. In summary, although high salinity strongly inhibited individual growth, P. australis achieved photosynthetic recovery based on structure-function coupling by increasing chlorophyll content, optimizing stomatal behavior, and enhancing clonal population density. This reflects its strong physiological plasticity and ecological adaptability in saline environments. The findings of this study provide insights into the dynamic regulation of carbon sequestration in coastal wetlands and support the development of carbon-negative strategies in saline-affected coastal regions.