This study was conducted to analyze the dynamics of carbon (C), nitrogen (N), and phosphorus (P), and their stoichiometric characteristics in different organs (leaves, branches, and roots) and during different seasons, in a young plantation of Platycladus orientalis in the Beijing Jiulong nature reserve. Studies on these relationship provide insights into interactions among plant functional traits and plant strategies for resource acquisition and mass partitioning. The results showed that branches contained the highest concentration of C but the lowest concentration of N. Leaves had the highest concentrations of N and P, whereas roots possessed the lowest concentration of C and P. Moreover, leaves had the lowest C:N and C:P ratios, whereas roots had the highest ratios of N:P and C:P, and branches had the highest C:N ratio but the lowest N:P ratio. Furthermore, during the growing season, the concentration of C in each organ was more stable than that of N and P. The C:N and N:P ratios in each organ were more stable than the C:P ratio. Both C and N concentration were strongly positively correlated with P concentration, with the change in C:P and N:P ratios being mainly determined by P concentration. Plant organs differed significantly in terms of the concentrations of C, N, and P and the ratios C:N, C:P, and N:P, whereas seasonal factors significantly influenced the concentrations of N and P, but their interaction effect only had influence on the concentration of P. Compared with the seasonal variations, the difference between organs made a larger contribution in determining the concentrations of C, N, and P and their stoichiometry in P. orientalis. The concentrations of C, N, and P and their stoichiometry showed no significant relationships among plant organs, except for the concentration of P and the C:P ratio between leaves and branches, which indicates that the organs have specific requirements for element absorption and utilization during the process of organ differentiation. The recorded leaf N:P ratio less than 14 indicates that the growth of young P. orientalis plants is more restricted by N during the growing seasons.