To investigate the differences of nutrient status and carbon (C) sequestration capacity between mixed forests and pure forests, the pure forests of Robinia pseudoacacia, pure forests of Pinus tabuliformis, mixed forests of Robinia pseudoacacia and Pinus tabuliformis, and mixed forests of Robinia pseudoacacia and Populus davidiana, located in Loess Hilly and gully region of China, were taken as the objects. Through the field investigation and indoor analysis, the ecological stoichiometry and C storage of the pure and mixed forests ecosystems were studied. The results showed that:(1) Compared with the pure forests, the mixed forests of Robinia pseudoacacia and Pinus tabuliformis significantly increased the C contents of branches and roots, the phosphorus (P) contents of leaves and trunks, the C:P and N:P ratio of branches of Robinia pseudoacacia species, and also significantly increased the nitrogen (N) contents of leaves, branches and roots and the N:P ratio of branches, trunks and roots of Pinus tabuliformis species, but significantly decreased the C:N ratio of all organs of Pinus tabuliformis species, while the mixed forests of Robinia pseudoacacia and Populus davidiana only significantly increased the P content of branches of Robinia pseudoacacia. (2) The content of soil C in the mixed forests of Robinia pseudoacacia and Pinus tabuliformis was significantly higher than that of the pure Robinia pseudoacacia forests, and the content of soil P was significantly lower than that of the pure Pinus tabuliformis forests. The differences of P in the soils between the mixed forests of Robinia pseudoacacia and Populus davidiana and the pure forests of Robinia pseudoacacia were not significant. (3) There was a significant correlation between the leaf and litter C contents in two pure forests. And the C:N, C:P ratios of the total pure forests were significantly correlated between tree leaf-litter-soil; but in total mixed forests, only P contents in litter and soil were significantly correlated with C:P ratio. (4) The C storage in tree layer of mixed forests of Robinia pseudoacacia and Populus davidiana was significantly higher than that of the pure Robinia pseudoacacia forests, and the C storage in understory vegetation layer and soil layer of mixed forests of Robinia pseudoacacia and Pinus tabuliformis was significantly higher than those in the pure Robinia pseudoacacia forests. This study could not only lay a foundation for an in-depth understanding of the nutrient cycle mechanism in the Loess Plateau, but also provide a theoretical basis for the management of plantation in the Loess Plateau.