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<i>Phragmites australis</i> (<i>P</i><i>. australis</i>) is one of the most important plant species found in wetland ecosystems, and its aboveground biomass (AGB) is a key indicator for assessing the quality or health of a wetland site. In this study, we combined Sentinel-1/2 images and field observation data collected in 2020, to delineate the distribution of <i>P</i><i>. australis</i> in the Momoge Ramsar Wetland site by using a random forest method, and further, to estimate AGB by comparing multiple linear regression models. The results showed that the overall classification accuracy of <i>P</i><i>. australis</i> using the random forest method was 89.13% and the <i>P</i><i>. australis</i> area in the site was 135.74 km² in 2020. Among various remote sensing variables, the largest correlation coefficient was observed between dry weight of AGB of <i>P</i><i>. australis</i> and Sentinel-2 red edge B₇, and between fresh weight of <i>P. australis</i> AGB and red edge B₅. The optimal models for estimating dry and fresh weight of <i>P. australis</i> AGB were multiple linear regression models, with an accuracy of 75.4% and 69.2%, respectively. In 2020, it was estimated that the total fresh weight of <i>P. australis</i> AGB in this Ramsar site was 21.2 × 10⁷ kg and the total dry weight was 7.2 × 10⁷ kg. The larger weight of <i>P. australis</i> AGB was identified mainly at central and western sites. The application of Sentinel-2 red-edge band for AGB estimation can significantly improve the model estimation accuracy. The findings of this study will provide a scientific basis for the management and protection of wetland ecosystems and sustainable utilization of <i>P. australis</i> resources.