Aims Phragmites australis, a typical perennial rhizomatic wetland plant, is an important species in marsh wetland ecosystems. However, with the global climate change and the gradual drying of wetlands, the growth of P. australis is often limited by soil moisture. Understanding the response of P. australis to changes in soil moisture can provide a theoretical basis for the protection and dynamic prediction of its community, as well as for research on the response mechanisms of wetland plants in heterogeneous habitats. Methods In this study, P. australis communities in three typical marsh wetlands in northern China, i.e., Daihai Wetland, Horqin Wetland and Qingtongxia Reservoir, were studied, and the effects of low and high soil moisture, and other environmental factors on the aboveground traits, belowground traits and their relationships of P. australis were analyzed. Important findings Compared with low soil moisture, high soil moisture significantly increased the aboveground biomass and specific leaf area, and significantly reduced the root to shoot ratio of P. australis. High soil moisture significantly reduced the contents of non-structural carbohydrates in leaves, and nitrogen and phosphorus in stems. High soil moisture significantly increased the root biomass, root surface area and root volume, and significantly decreased the root diameter of P. australis. Soil moisture did not affect the positive correlation between root biomass and stem non-structural carbohydrates content. However, the decrease in soil moisture reversed the negative correlation between root diameter and leaf nitrogen content under high soil moisture, as well as the positive correlations between root diameter and leaf non-structural carbohydrates content, and between root diameter and stem non-structural carbohydrates content. In high soil moisture areas, total soil nitrogen and total soil phosphorus were important factors that affected the aboveground and belowground traits of P. australis; in low soil moisture areas, temperature and precipitation were also important factors. Our results indicate that soil moisture may indirectly affect total soil nitrogen and total soil phosphorus and temperature to change the correlations between some aboveground and belowground traits. In summary, high soil moisture was beneficial to the growth of aerial parts and roots of P. australis, but it reduced the contents of nitrogen and phosphorus in stems and the content of non-structural carbohydrates in leaves. soil moisture influences the aboveground and belowground traits and their relationships in P. australis, by indirectly affecting soil nitrogen and phosphorus contents, as well as temperature.