Phragmites australis is regarded as an important cosmopolitan species that has the potential to enhance the structure, function, and stability of the Yellow River delta, China. This study investigated the responses of Phragmites australis to various hydrologic and salinity conditions with a focus on plant growth, leaf morphology, biomass and allocation, gas exchange, and chlorophyll fluorescence physiology. The plants were subjected to a full factorial design with three soil salinity levels, two levels of rainfall amounts, and two watering frequencies. Results from a 62-day experiment revealed that both higher soil salinity and lower rainfall amounts greatly reduced plant growth, average leaf area, and photosynthetic characteristics (e.g., net photosynthetic rate, transpiration rate, and stomatal conductance). The interactive effects of soil salinity and rainfall amounts were found on most gas-exchange parameters and biomass parameters. Lower rainfall frequency reduced total chlorophyll concentration, intercellular carbon dioxide, and stomatal limitation value, but the total biomass and biomass of each part were not affected. Although it may suffer from higher salinity and drought in the long term, Phragmites australis will continue to dominate wetland communities of the Yellow River delta, at least in the short term.