Abstract: Phragmites australis grown in the transitional belt between the desert and oasis of dry areas in the lower reaches of the Tarim River in Xinjiang was studied. Meteorological factors, soil moisture, soil salt and leaf water potential of P. australis were measured to analyze daily changes of leaf water potential of P. australis grown with various depths of groundwater. Environmental factors dominating water potential were examined, and they indicated thresholds that reflected water potential changes. 1) When the groundwater became deeper, the daily peak of leaf water potential was reduced; 2) When the groundwater became deeper, the 0-110 cm soil layers became dryer, and P. australis horizontal roots distributed into deeper soil layers; 3) In arid areas, the survival strategy of P. australis appeared to be “higher soil moisture benefits population expansion, while lower soil moisture promotes the height of individual plants”; 4) Soil moisture and air temperature were the dominant factors influencing the change of leaf water potential. P. australis prefers soil containing 10%-20% of water. The threshold of soil moisture at which leaf water potential changed was 18.07%, and the threshold of air temperature was 33.48℃. Leaf water potential did not change if soil water was>18.07% or air temperature was below 33.48℃. This study provides a theoretical basis for mitigating drought stress of P. australis and maintaining its normal growth.