Knowledge of seasonal variations of carbon (C), nitrogen (N), and phosphorus (P) concentrations plus C:N:P ratios can provide insight into the dynamics of biological processes, nutrient cycling, resource acquisition, and mass partitioning in plants and soils. Platycladus orientalis (Cupressaceae) is an important tree species distributed throughout northern China. We studied the seasonal dynamics of C, N, and P concentrations and their stoichiometry in leaves, branches, roots, and soil in a P. orientalis plantation. Plant and soil C, N, and P concentrations, and their stoichiometry varied significantly among ecosystem components and growth season sampling times, indicating nutrient allocation patterns and mobility in the P. orientalis plantation. Accurately quantifying and comparing C:N:P stoichiometry in the plant and soil of P. orientalis requires a study of plant organs, soil depth, and sampling times. The mean N concentration in leaves was 14.06 g kg^-1 and the mean N:P ratio was 11.82, indicating that the growth of P. orientalis was N limited during the growing season. N and P concentrations and stoichiometry in plants were correlated with those in soil, suggesting coupled nutrient control between plant and soil. The C:N:P ratio was regulated in different components of the ecosystem.