Abstract: In order to investigate the effect of root competition on production in mixtures of Onobrychis viciifolia and Bromus inermis, plant spacing and population density effects were varied so as to manipulate the underground competitive environment. The geometric morphology, topological structure and fractal characteristics of the root system were used to analyze and compare the changes in root structure of the two forage species in the competitive environment. The results showed that: 1) Mixed species sowing in alternate rows or increasing row spacing increased aboveground biomass of both forage species, and the root biomass and root:shoot ratio of B. inermis were also increased, while the root biomass and root:shoot ratio of O. viciifolia were not obviously changed. 2) Mixed species seeding within rows, and spacing increases resulted in increased formation of fine root (diameter≤0.16 mm) in B. inermis, and of roots in the category (diameter≤0.16-0.50 mm) of O. viciifolia. Thus the root length, root surface area and ratio of root length to root biomass increased for both species. Mixed species sowing within rows with a root barrier between, resulted in increased root diameter of both forage species, so that the root volume and ratio of root surface area to root biomass increased. 3) Under different mixed seeding patterns, the topological index (TI) of both forages was close to 1, the fixed topological index (q_a) was greater than 0.5, and the branching structures were of the fishtail shape. With increasing space between rows, there was a tendency for the topology of the roots of both forages to adopt the forked branch topology. 4) Root length, root surface, root biomass and aboveground biomass had a linear positive correlation between the two forage species, and there were significant correlations between root biomass and other root morphology parameters, and no significant correlations between root biomass and root:shoot ratios. Therefore, row spacing, planting pattern of species within rows, and population density effects changed the competitive interaction between the underground root systems of mixed B. inermis and O. viciifolia stands. Plasticity of root morphology was the ecological adaptation strategy that enabled effective utilization of water and nutrients, and more effective exploration of the root growth space.