The aims of this study were to reveal the allometric relationships between population density, plant morphological characteristics, and organ biomass allocation, and to elucidate the adaptive strategy of the plant during the restoration of degraded land. Specifically, we used allometric analysis to evaluate the effect of density on morphological characteristics and the distribution of the biomass of organs in Corispermum macrocarpum. Our results showed that density strongly affects the plant architecture. With an increase in density, the height of C. macrocarpum decreased, and the branch number and length clearly decreased. The biomass of all organs decreased significantly with an increase in density. With an increase in the population density, the biomass allocation was decreased to stem and reproductive organs, but increased to the root and leaf organs. These findings are consistent with the prediction of optimal allocation theory in the context of moisture, nutrient, and light limitation. The allometric relationship of height vs. root biomass was significantly influenced by density. Changes of root vs. above-ground biomass, leaf vs. root, and reproductive organs vs. root were not related to population density, indicating a state of ‘apparent plasticity.’ The allometric relationships of root vs. stem, stem vs. above-ground biomass, leaf vs. other organs, and reproductive organs vs. other organs were significantly affected by population density, showing a state of ‘true plasticity.’ Our results suggest that population density changes the ontogenesis system of plant architecture in C. macrocarpum, and can indeed affect the allometric relationships of organs, resulting in a trade-off in biomass allocation among organs to complete the life history.