The aim of present study was to investigate the adaptability of Phragmites australis (common reed) under three different water and salt levels in the field. Environmental factors, photosynthetic parameters, osmotic adjustment substances, and antioxidant enzymes were determined in P. australis, which were grown in swamp, light salt meadow, and dune habitats. P. australis grown in light salt meadow and dune habitats showed two peaks of diurnal variation of P_n, which indicated a remarked photosynthetic "midday depression", while there was one peak in swamp reed. The midday depression of photosynthesis was intrinsically caused by the reduction of stomatal conductance. As critical indices of resistance to drought and salinization, proline and soluble sugar content in P. australis leaves increased significantly with the intensification of drought and salt, and soluble sugar was more important for the resistance to drought and salinization. Superoxide dismutase (SOD) and catalase (CAT) activities in P. australis leaves increased significantly when there was less water and more salt in the root region. The responses of SOD and CAT to water deficiency were more sensitive than to salinity, and could effectively resolve oxidative damage induced by the water deficit. The peroxidase (POD) activities were high in both the dune and saline habitat, indicating that POD plays a major role in resistance to water deficit and salinity in oxidative defense systems of P. australis. The malondialdehyde (MDA) content was the lowest in swamp reed, and highest in light salt meadow reed.