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Wetlands play an important role in global carbon (C) cycling. However, little is known about the effects of climate warming on wetland C cycling, which may bias our prediction of wetland functioning under global climate change. This study was undertaken to examine the potential changes in soil C, nitrogen (N) and phosphorus (P) pools and the related microbial activities after 7 years of in-situ experimental warming by open-top chambers (OTCs) in a Phragmites wetland in the Yangtze Estuary. The mean soil temperature, moisture, and salinity in both the 0-10 cm and 10-40 cm soil layers were significantly increased by the warming treatment. In the upper 10 cm soil layer, significant increases in total organic C (TOC) and total N (TN) were observed under the warming treatment, while the phenomenon were further clarified by comparing the year-round data in the warming plots between 2008 and 2015; However, the molar C/molar N ratio (C/N) and total phosphorus (TP) were not affected. In both layers, microbial biomass C (MBC) and the molar MBC/molar MBN ratio (MBC/MBN) showed no significant responses, but microbial biomass N (MBN) and microbial biomass P (MBP) were increased by warming. Warming did not affect MBC/TOC, but it significantly increased both MBN/TN and MBP/TP throughout the 0-40 cm soil profile. Invertase activity was not affected in the 0-40 cm soil profile. The acid phosphatase, alkaline phosphatase, and urease activities in the upper 10 cm soil layer were increased by warming. Only alkaline phosphatase activity was increased by warming in the 10-40 cm soil layer. These field-experimental warming results suggested that in temperate P. australis wetlands without tidal impacts, climate warming might increase the soil organic C pools while exerting no effect on microbial C decomposition, whereas the N and P cycling that is mediated by soil microbes might be enhanced by moderate warming.