Analyzing carbon (C), nitrogen (N), phosphorus (P), and their stoichiometric characteristics among coexisting plant organs and between species in artificial mixed plantations not only elucidated plant nutrient allocation strategies and environmental response mechanisms but also played a crucial role in constructing stable communities within regional artificial plantation ecosystems. This study focused on typical artificial mixed plantations in the Loess Hilly Region, including mixed plantations of Hippophae rhamnoides and Pinus tabuliformis (H_rP_t), H. rhamnoides and Robinia pseudoacacia (H_rR_p), as well as pure plantations of H. rhamnoides (H_r), P. tabuliformis (P_t), and R. pseudoacacia (R_p). Utilizing ecological stoichiometry methods, we analyzed the stoichiometric traits of C, N, and P in leaves, branches, and fine roots. We quantified intra-organ and interspecific differences using the coefficient of variation (CV) and examined allometric relationships (C-N, C-P, N-P) through allometric equations and slope tests. The results showed that: (1) Compared with P_t, the contents of N and P in H_rP_t(P_t) leaves increased by 8.42% and 19.65% respectively, and C ∶ P decreased by 14.58% (P<0.05); Compared with R_p, the C contents of leaves, branches and fine roots of H_rR_p(R_p) decreased by 8.94%, 4.60%, and 6.64% respectively (P<0.05); Compared with H_r, the N content of fine roots in H_rP_t(H_r) and H_rR_p(H_r) decreased by 9.81% and 11.52% respectively (P<0.05), indicating that the mixed plantation of H. rhamnoides and P. tabuliformis enhanced the N and P acquisition capabilities of P. tabuliformis through N sharing, while the mixed plantation of H. rhamnoides and R. pseudoacacia formed N redundancy, and the C distribution of R. pseudoacacia shifted from storage to symbiosis. (2) Compared with P_t, the CV of C content in H_rP_t(P_t) leaves decreased; compared with R_p, the C content CV in the leaves, branches and fine roots of H_rR_p(R_p) increased (P<0.05). Compared with H_r, the CV of C content in H_rP_t(H_r) leaves decreased, and the CV of P content in H_rR_p(H_r) branches and C ∶ P, N ∶ P in fine roots decreased (P<0.05). This indicated that mixed plantation of H. rhamnoides and P. tabuliformis was beneficial to improving the internal stability of C in the ecosystem, while mixed plantation of H. rhamnoides and R. pseudoacacia helped improve the internal stability of C in H. rhamnoides, but reduced the internal stability of C in R. pseudoacacia. (3) The C-N of branches and fine roots of P_t and H_rP_t(P_t) showed negative allometric growth (P<0.05); The leaf C-N-P and fine root C-N of R_p and H_rR_p(R_p) showed positive allometric growth, the branch N-P of R_p showed negative allometric growth (P<0.05), and H_rR_p(R_p) showed isometric growth. The C-N of H_r leaves showed negative allometric growth, C-P showed isometric growth, and N-P showed positive allometric growth (P<0.05), while the C-N of H_rP_t(H_r) leaves showed isometric growth, leaf C-P and branch N-P showed positive allometric growth (P<0.05), and the C-N and branch N-P of H_rR_p(H_r) leaves showed isometric growth. This indicated that the mixed plantation of H. rhamnoides and P. tabuliformis was conducive to coordinating the distribution of C and N in the branches and fine roots of P. tabuliformis and enhanced the synergy of P in H. rhamnoides, while the mixed plantation of H. rhamnoides and R. pseudoacacia only alleviated the negative heterocyst contradiction of N and P in R. pseudoacacia branches. In conclusion, the mixed plantation of H. rhamnoides and P. tabuliformis exhibited a more stable and efficient interspecific resource complementarity mechanism through nutrient synergy. The research results could provide a scientific basis for formulating and optimizing nutrient regulation strategies in regional plantation ecosystems.