Phragmites australis is a typical clonal plant of long-rhizome, and its natural populations rely mainly on vegetative propagation of rhizomes for renewal. By sampling the soil from a unit area, with method of classifying the age classes of rhizomes according to the actual living years, a field investigation was performed on age structure of rhizome of different age classes of P. australis population in Zhalong wetland. The results showed that the rhizome modules of P. australis populations in four habitats were all composed of five age classes (2nd-6th) in June, and six age classes (1st-6th) from July to October. From June to October, the ratio of rhizome length of 1st was gradually increasing, but the ratio of 2nd and 3rd decreased slightly in three habitats except for the slight increase in saline-alkali habitat. At the same time, the ratio of 4th-6th gradually decreased with the largest ratio of 3rd, followed by 2nd, 4th, 1st, 5th, and the 6th was the smallest. The ratio of rhizome biomass of 1st-3rd was gradually increasing, while gradually decreased between 4th-6th with the largest ratio of 3rd, followed by 4th, 2nd, 5th, 6th, and the 1st was the smallest. The ratio of rhizome length and biomass of different age classes and the actual growth time after turning green were consistent with a linear function (R²>0.91,P<0.01;R²>0.81,P<0.05). The rhizome length showed a declining age structure from June to September, while transformed into a stable age structure in October. The rhizome biomass showed a declining age structure during the whole growing season. The status and function of rhizome module of different age classes were different in the P. australis populations. The age structure of rhizome module contains important information on population regulation.