Abstract: Coastal waters face long-term risks of eutrophication, posing a serious threat to the local ecological environment. This study focused on a Phragmites australis salt marsh on the northern shore wetland of Hangzhou Bay. Using an in-situ tidal simulation experiment system, we investigated the effect of salinity—a critical environmental factor—on the growth and nitrogen removal function of P. australis. Our data revealed that P. australis exhibited optimal growth and nitrogen removal capacity at a salinity of 5, with a total biomass of (2.62±0.36) kg/m² and the highest nitrogen removal rate of (73.83±23.09) mg/(m²·d). With salt stress intensifying, the growth of P. australis was inhibited, accompanied by a decline in the root-to-shoot ratio and soil nitrogen density, as well as a reduction in the relative abundance of denitrifying microorganisms, leading to a significant decrease in the nitrogen removal rate. The findings provide a theoretical basis and data support for the ecological restoration of P. australis salt marsh and eutrophication control.