Elevated atmospheric CO₂ concentration and sea level can influence the stability of the wetland by affecting the distribution and growth of plants. Underground biomass is a critical factor regulating ecosystem functions of coastal marshes, including the accumulation of soil organic matter and the maintenance of wetland elevation. In this paper, OTC (open top chamber) experiments were set up to explore the varied responses of three communities with different plant compositions (SC community: C₃-dominated community; MX community: mixed C₃-C₄ community; SP community: C₄-dominated community) to elevated CO₂ and sea level. The results show that the increase of CO₂ significantly promotes the rhizome, root and total underground biomass of SC, MX and SP communities, and the biomass varies greatly between years. The rise of sea level significantly reduces root biomass of plants in three communities and total underground biomass under elevated CO₂ treatment in SC community and ambient CO₂ treatment in SP community, but has no significant effect on rhizome biomass. Under the condition of high salinity, elevated CO₂ alleviates the stress of high salinity on plants to some extent, but the fertilization effect of elevated CO₂ decreased. The total underground biomass of SC and MX communities under ambient CO₂ treatment is decreased over time, which is mainly due to the rise of sea level. Increased total biomass under elevated CO₂ partially slows down the decrease and offsets the stress to some extent. Therefore, the rise of sea level may affect the stability of wetlands in the future, and elevated CO₂ could alleviate the impact of some sea level rise on wetlands.