To derive a deeper understanding of the ecological mechanism of the spread of Sophora alopecuroides, this study systematically analyzed C, N, and P stoichiometry and its seasonal dynamic changes in the roots, stems, and leaves of Sophora alopecuroides distributed in the Yili River Valley. The results showed average C, N, and P contents to be 391.40, 13.17, and 1.51 mg/g; the average ratios of C/N, N/P, and C/P were 45.61, 8.52, and 326.38, respectively. The changes of the mean C, N, and P content showed the same pattern in the roots, stems, and leaves of Sophora alopecuroides during the completely growing season; leaves contained most of the nutrients, followed by stems and roots. However, they exhibited different patterns in the stoichiometric analysis. The C content increased with increasing plant growth, while N and P content declined. The content of C, N, and P in the root system of Sophora alopecuroides had a tendency to decline with the change in depth. The differences between organs indicated that the absorption and utilization of C, N, and P were specific to the plants during different growth periods. Plant leaves tended to have low C, N, and P content and N/P ratio. The growth of Sophora alopecuroides was limited by N and P jointly, but will be easier to constrain by N elements.