In this study, poplar fine roots in two successive rotation plantations were sampled over seasons. Root samples were grouped from first to five orders to examine the seasonal dynamics of carbon and nitrogen contents of poplar fine roots with orders, and compared their differences between two successive rotation plantations, and finally to find the relationships between the fine root growth and the productivity decline of successive rotation poplar plantations. The results showed that nonstructure carbohydrates (NSC) content increased significantly with root orders, while nitrogen content decreased. The contents of total carbon and NSC were significantly related to total nitrogen content. Root orders explained 98.2% variance of carbon and nitrogen contents of poplar fine roots, and the difference between rotations only explained 1.7% of variance. Poplar fine roots consisted of more carbon and less nitrogen with root orders, and the seasonal changes in contents of total carbon, total nitrogen and NSC showed significant difference between rotations, while that of the C:N ratio didn’t show significant difference. Root order and season showed interaction effect on carbon and nitrogen dynamic. The C:N ratio was about 20:1 in lower order roots,  and more than 30:1 in higher order roots. The C:N ratio in summer and autumn was significantly less than those in other seasons, while NSC content was the highest in November. This study indicated that the allocation of carbon and nitrogen in fine roots was closely correlated with fine root orders. Both NSC content and C:N ratio were of greatly important ecological significance in fine root turnover and growth regulation.