Genotypic diversity is an important part of biodiversity, which plays an important ecological role not only at the community level, but also at the population level. In this study, we set up five gradients of genotypic diversity to explore the effects of genotypic diversity on Leymus chinensis population from the perspective of age structure. The results showed that the biomass and number of 2nd age class tillers of L. chinensis population increased significantly with the increase of genotypic diversity (P<0.05),and the biomass of rhizome,length and dry matter accumulation of L. chinensis in different ages increased significantly (P<0.01). At five gradients of genotypic diversity,percentage of biomass and tiller number of 1st or 2nd age class tillers were the highest among different ages and showed expanding or stable age structure. But percentage of rhizome length and biomass in L. chinensis population of 2nd age rhizomes were highest proportion among different ages and showed stable age structure. With the increase of genotypic diversity,the age structure of tiller number and biomass of L. chinensis population transited from expanding type to stable type,but the age structure of rhizome length and biomass were stable type. The characteristics of tillers and rhizomes at age 4th of 12 genotypic diversity level were higher than other genotypes. The cumulative length of the rhizome of L. chinensis not only reflects the asexual reproduction ability, but also reflects its spatial expansion ability. High genotypic diversity significantly increases the length of the rhizome of L. chinensis population. The reason is that genotypic diversity can improve the soil nutrient absorption efficiency of L. chinensis, thus effectively promoting the root growth and providing conditions for population expansion. Therefore,the tillers and rhizomes of L. chinensis population have an expanding or stable age structure at different genotypic diversity levels. Moderate genotype number promotes the increase of the biomass and number characteristics of tillers and rhizomes at different age levels. This is due to the contribution of complementary effect to the positive effect of genotypic diversity. There are some differences in morphological and physiological characteristics among different genotypic levels of L. chinensis, resulting in different nutrient requirements and resource utilization ability of different genotype of L. chinensis. The increase of genotypic diversity can improve the utilization rate of resources and promote the development of L.chinensis population.However, over high genotypic diversity will increase the proportion of senior tillers and rhi senior zomes in the population of L. chinensis,and the development of population showed a decline signal.