Total nitrogen content， total nitrogen isotope （δ¹⁵N）， and free amino acid concentration in leaf of Phragmites australis （Cav.） Trin. ex Steud. under flooding for 0-11 d were studied. The results show that with the increase of flooding days， there is no significant variation in total nitrogen content in leaf of P. australis， δ¹⁵N value shows a gradual tend to be negative， and concentrations of total free amino acids， Ser， Asp， and Asn first increase and then decrease. Under flooding for 1 d， δ¹⁵N value decreases obviously and free amino acid concentration increases obviously compared with those under flooding for 0 d. It is indicated that δ¹⁵N value and free amino acid concentration are good indicators of P. australis under flooding condition.