Groundwater, as the main form of available water in arid and semi-arid areas, is the primary ecological factor affecting the survival of vegetation. In this paper, the ecological response and adaptive mechanism of Phragmites australis under different groundwater levels were discussed using artificial simulation of groundwater levels. The results showed that with a decrease in groundwater level:(1) Net photosynthetic rate (P_n), transpiration rate (T_r), stomatal conductance (G_s), and intercellular CO₂ concentration (C_i) tended to increase first and then decrease, and the growth season exhibited a single peaked curve, which peaked from July to August; (2) Chlorophyll (Chl) content increased first and then decreased; (3) The change in proline (Pro), soluble sugar (SS), and soluble protein (SP) content, the osmolytes of cells in P. australis, was inconsistent with the variation in groundwater levels and the growing season, but there was a mutual compensation relationship among them in the process of inhibiting drought stress; (4) Malon dialdehyde (MDA) content exhibited a trend of first increasing and then decreasing, and the growth season was significantly increased; superoxide (SOD) and catalase (CAT) activity increased significantly; and SOD was more sensitive than CAT to drought stress, which is the main antioxidant enzyme that adapts to drought stress; (5) For P. australis, the order of importance of its physiology factors to P_n was G_s > SP > Chl > T_r > C_i > SS.