C:N:P stoichiometry is widely used in ecological research, but its role in the allocation of plant biomass is unclear. To explore this issue, we collected soil and alfalfa samples from 1, 2, 3, 5, 7, 9 and 10 year of alfalfa (Medicago sativa) plantations on the Mu Us Desert, China. We examined the aboveground and underground biomass of each plantation, the C, N, P contents, stoichiometric ratio of plants and soil, and soil biological characteristics. The C content of alfalfa increased first and then decreased with the growth age, which was opposite to N and P content, their most significant changes were in the 7-year stand. The individual changes of C-N-P in alfalfa led to the C:N and C:P increased first and then decreased, N:P increasing with the growth age. Soil OC, TN, C:P and N:P were increasing with the stand age, and TP content and C:N were increasing first and then decreasing. The C contents, C:P and N:P between alfalfa and soil were strongly correlated. N:P and underground biomass were positively correlated, while negatively correlated to aboveground. Our results show that the nutrient content and stoichiometric ratio in soil and plants were closely associated, after 7 years planting, the alfalfa’s growth was limited by P, it responds to the increased N:P by increasing root-shoot ratio, and soil fertility had also been improved at same time. Overall, these results provide a reference for further research on stoichiometry as an indicator to alfalfa yields on the Mu Us Desert.