C, N, and P are the basic chemical elements required for plant growth and regulation, and their balance and stability play an important role in many physiological functions. Ecological stoichiometry combines the first law of thermodynamics, the theory of evolution by natural selection, and the central dogma of molecular biology, and has organically integrated biological studies at the scales of molecules, cells, species groups, communities, and ecosystems. Plants adjust their growth rate to adapt to environmental changes throughout their lives. The growth rate can be regulated by C:N:P ratios; therefore, these ratios reflect the adaptability of plants to the external environment. Seasonal variations in C:N:P ratios reflect the seasonal variations in plant growth rate. Consequently, research on the seasonal variations in C:N:P ratios can increase our understanding of plant adaptation strategies. There has been limited research on the change in C, N, and P during the xerophyte growing season. In this study, Phragmites australis, the common reed, was used to examine the contents of C, N, and P and their ratios in roots, stems, and leaves in different seasons in the Keriya Oasis, China. The contents of C, N, and P in the plants ranged from 140.4 to 565.1 (average 393.36±99.92) mg/g for C, 2.19 to 34.48 (average 12.43±8.70) mg/g for N and 0.26 to 4.42 (average 1.25±0.74) mg/g for P. The means of the C:N, N:P, and C:P ratios were 54.55, 9.96, and 441.27, respectively. The order of the mean contents of C, N, and P in all organs was leaves > stems > roots during the growing seasons. The content of C in roots, stems, and leaves increased first, and then decreased again. The contents of N and P in roots and stems decreased gradually during the growing season. The contents of N and P in leaves decreased first, and then increased again. The changes in C:N and C:P ratios were opposite to those in corresponding N and P contents in roots, stems, and leaves. The ratio of N:P was highest in leaves and was lowest in roots. The C, N, and P contents and their ratios had moderate variability in all organs throughout the growing season. Factorial analysis of variation for each parameter, with sampling season and organs as independent variables, showed that the contents of C and P and C:N and C:P mass ratios mainly depended on the season. The content of N and the N:P mass ratio were mainly determined by the organs. Furthermore, N is a restrictive factor for Phragmites australis in the Keriya Oasis during its growth and development.