Plant growth characteristics have plasticity in response to changes of environmental conditions, and different environmental factors have different effects on plant plasticity. The allometric method was used to analyze the morphological-structural traits and biomass allocation strategies by simulating population densities of S. collina (16, 44, 100 and 400 plants/m²) during the restoration of degraded grassland. The results showed that the increased population density could inhibit the growth of S. collina. population density had significant effects on plant height, root length, primary branch number, second-level branch number, third-level branch number, and total branch length, indicating that population density changes have caused significant changes in vertical and lateral growth of S. collina. Population density changes also influenced on the biomass of plant. The biological relationships between roots, stems, and leaves are an "apparent plasticity", and the growth strategy has not changed at all, only the variation of biomass allocation were caused by the change of individual plant size. Changes in plant height, total branch length, primary branch number and reproductive biomass allocation were significantly influenced by population density, representing "true" plasticity. The results showed that variation of population density caused the strategy change of the reproductive allocation rather than the leaf allocation, indicating that when environment changes, plant would adjust the reproductive strategy to adapt to surrounding environmental factors, to ensure the survival and reproduction of the population.