As the components having the highest biological activity and fastest turnover rate in the plant root system, fine roots (≤2mm) are very sensitive to global climate change and significantly affect the carbon balance and nutrient cycling in the belowground part of the terrestrial ecosystem. Coastal wetlands are among the ecosystems with significant carbon sink functions in the earth, where the belowground part plays a vital role. Up to date, several studies have revealed the influence mechanism of global warming on the above-ground part of coastal wetlands, however, relatively few studies were conducted to investigate the influence of global warming on the underground part of coastal wetlands. Particularly, until now, how will the fine roots of coastal wetland plantsl respond to warming conditions is unclear. This study was conducted to examine the potential effects of climate warming on fine roots in the Phragmites australis wetland in Chongming Dongtan of the Yangtze River Estuary. The open-top chamber (OTC) method was used to simulate a warming climate. From May to October 2019, the methods of minirhizotron and soil auger were used to determine the total length, total surface area, specific root length, specific root surface area, average root diameter, and biomass of the fine roots in the 0-40 cm soil layer (i.e., the fibrous roots of P. australis with the diameter ≤ 2 mm). The net growth rate and turnover rate of the fine roots were also calculated. The results showed that, during the entire growing season, warming significantly increased the total length, total surface area, and total biomass of the fine roots. The increases of the above indicators were more apparent in the 0-20 cm soil layer compared to the 20-40 cm soil layer. Warming did not have a significant impact on the specific root length and specific root surface area. Significant enhancement of the net growth rate of the fine roots caused by warming occurred in the entire 0-40 cm soil layer, with increased seasonal variability. Warming significantly increased the turnover rate of the fine roots in the 10-40 cm soil layer, whereas it didn't change their turnover rate in the 0-10 cm soil layer. In summary, in the Phragmites australis wetland, warming significantly increased the quantity and growth rate of the fine roots, changed their distribution pattern in the 0-40 cm soil layer, but it did not affect the efficiency of the fine roots to absorb soil water and nutrients. The increase in the turnover rate and the biomass of the fine roots might lead to significantly increased root-derived organic carbon input, especially in the 0-20 cm soil layer, which might profoundly affect the soil carbon balance of the Phragmites australis wetland. Our results indicated that the potential changes of plant fine root-related carbon processes under warming conditions should be fully considered in the simulation and prediction of soil carbon balance in coastal wetlands.