Wetlands have strong purification capacity by reducing pollutants (such as nitrogen and phosphorus) in water. We measured the concentrations of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (DIP) in the porewater across four seasons at three habitats: tidal flat, Scirpus mariqueter and Spartina alterniflora in the Nanhui Shore of Yangtze Estuary. We analyzed the relationships between nutrient reduction and environmental factors in the sediment. Results showed that the concentrations of DIN and DIP in the porewater of two vegetated habitats were significantly lower than that in the tidal flat (P<0.05). The DIN relative reduction efficiency was 82.5%-97.3% in the S. alterniflora habitat, followed by S. mariqueter habitat with 25.5%-75.8%. The DIP relative reduction was 54.1%-83.9% in the S. alterniflora habitat, followed by 55.7%-76.4% in S. mariqueter habitat. The concentrations of NH₄⁺-N and PO₄^3--P in the porewater increased with depth while NO₃^--N concentration showed an opposite tendency. The DIN concentrations were higher in winter than that in summer, while DIP concentrations were the highest in summer. Purification capacity was primarily regulated by sediment temperature, inorganic nitrogen (IN), available phosphorous (AP), and C/N ratio. The results would help improve our understanding of water purification services provided by salt marshes.