In order to explore the seasonal variations of carbon(C), nitrogen(N) and phosphorus(P) content and their stoichiometric ratios of the hydrophytes, we chose four typical emergent hydrophytes, Cyperus alternifolius, Phragmites australis, Lythrum salicaria and Scirpus validus in Jinze Reservoir. The hydrophytes were collected and analyzed in the different seasons from October 2017 to June 2018. The results indicated that: (1) The C, N and P contents of the four hydrophytes varied in the different ways. The C content of C.alternifolius and P. australis didn't show any significant variations among the four seasons, while that of L. salicaria and S. validus decreased significantly in the winter. The N content of the three plants except C.alternifolius decreased significantly in the early growing stage in spring and increased significantly in the summer, while that of C.alternifolius didn't show any significant variations in four seasons. The P content of S. validus didn't show any significant seasonal variations, while that of L. salicaria and P. australis decreased significantly in the winter, and that of C.alternifolius and P. australis decreased significantly in the summer. (2) The C/N/P ratio of the four hydrophytes had different seasonal variation characteristics. The C/N ratio of C.alternifolius didn't show any significant seasonal variations, while that of the other three hydrophytes increased significantly in the spring. The C/P ratio of P. australis and L. salicaria decreased significantly in the spring, and that of C.alternifolius and P. australis increased significantly in the summer, while S. validus didn't show such significant seasonal variations. To the N/P ratio, C.alternifolius appeared to have no significant seasonal variations, while the other three increased significantly in the winter when their biomass was lower and decreased significantly in the spring and then increased significantly in the summer as their biomass increasing. (3) The effects of the season on C, P content and N/P ratio of the four hydrophytes were much greater than those of the species. To the N content, C/N and C/P ratio, the effects of the species were slightly greater than those of the season. (4) The C/N and C/P ratio of the four hydrophytes shew the significant negative correlations with the N and P content respectively, which indicated that C/N and C/P ratio of the hydrophyte were determined by the N and P content respectively. There were no significant correlations between C and N content, and also C and P content of the hydrophytes in the autumn and summer, which indicated that the activity of C was independent of N and P in the summer and autumn. The N and P contents were significantly correlated only in the spring and summer. From above all, we got the conclusion that the different hydrophytes had the different seasonal variation characteristics in the C, N, P contents and corresponding stoichiometric ratios. Future studies need to synthesize the habitat factors to reveal the mechanisms laying behind this.