Abstract: Density effects of one year old Caragana microphylla seedlings were studied to reveal the interaction law among population's individuals formed in varied density. In the individuals and populations of C.microphylla, traits such as phonological traits, growing status, accumulation and allocation of biomass were studied under different density.Results showed that density had a certain effect on phenological traits and growing status of C.microphylla seedlings.The development of lateral branch germination retarded and the number of lateral branch decreased with the increase of density, and also, growth indicators above ground and underground all decreased, most of them reached an obvious level of the differences among different density treatments.Individual biomass decreased and population biomass increased with the increase of density.Individual biomass proportions of root, stem and leaf were 40%, 35% and 25% respectively, and biomass proportions of root and stem decreased and that of leaf increased with the increase of density.