Abstract: As one of the three major terrestrial ecosystems, wetlands play a critical ecological role in water conservation, climate regulation, and biodiversity maintenance. However, under the dual pressures of climate change and human activities in recent years, existing wetlands are facing increasingly severe threats, particularly from biological invasions that cause significant damage to wetland ecosystems. Cirsium pendulum, a perennial herbaceous plant of the Asteraceae family, characterized by its spiny body, has been extensively observed in the wetlands of the Sanjiang Plain, severely affecting the growth and regeneration of native vegetation. However, research on the ecological effects of C. pendulum spread in wetlands was scarce, which limited the development of effective wetland management strategies. This study, conducted in the natural wetlands of the Sanjiang Plain, analyzed the impacts of different spread levels of C. pendulum on the stability and species diversity of Deyeuxia purpurea communities through field plot surveys. The results showed that C. pendulum spread altered the species composition of D. purpurea communities, the species richness of the heavily spread community decreased from 17 species to only 7 species. Nine species, including Persicaria hastatosagittata, Viola patrinii, and Thelypteris palustris var. pubescens, were absent in heavily spread communities. Compared with non-spread communities, Patrick’s richness index and Shannon-Wiener diversity index were significantly lower in heavily spread communities, while Simpson’s dominance index and Pielou’s evenness index were significantly higher. As the spread intensity increases, the stability of the D. purpurea community showed a downward trend, with the stability of heavily spread communities reduced by 46% compared to the non-spread community. The spread of C. pendulum significantly reduced species diversity and community stability in the D. purpurea community, and caused notable changes in species composition. This study is of great significance for predicting the dynamics of wetland vegetation in the Sanjiang Plain under the context of global change and provides a scientific basis for formulating strategies for wetland vegetation protection and management.