This study measured the C, N, and P concentrations in the leaves, litter, and soil of plantations (Robinia pseudoacacia, Populus simonii, and Pinus tabuliformis) and natural secondary forests (Quercus wutaishanica, Quercus acutissima, and Betula platyphylla) in the hilly-gully region of the Loess Plateau by combining field investigations with laboratory analyses. We explored the ecological stoichiometry characteristics of leaf-litter-soil interactions in different forest types in order to understand the nutrient supply-demand status of different forest ecosystems. The results showed that 1) for leaves and litter, the C concentration was higher in plantations than in natural secondary forests, whereas the concentrations of N and P were higher in natural secondary forests than in plantations. The C:N and C:P ratios of the leaves and litter were higher in plantations than in natural secondary forests. 2) The significant differences between C, N, and P regarding the stoichiometric ratios of soil were mainly concentrated at the soil surface (0-10 cm) in plantations, while in natural secondary forests these differences were found deeper in the soil layer (10-50 cm). With an increase in soil depth, the C, N, and P concentrations gradually decreased. 3) The N concentration in the leaves showed significant positive correlation with the litter in the plantations; the N concentration of litter showed significant positive correlation with the soil in the natural secondary forests, and the C:P ratio in the leaves showed significant negative correlation with that of the soil (P < 0.05); and there were no significant differences in the remaining indicators. These results revealed that the growths of R. pseudoacacia and Q. wutaishanica were limited by P, other trees were restricted by N, the decomposition rate of litter was higher in plantations than in natural secondary forests, and the soil P was higher in plantations than in natural secondary forests. The results of this study could provide a theoretical basis for vegetation restoration and reconstruction in the Loess hilly-gully region of China.