Abstract: The positive interaction between plants of the artificial grassland ecosystem was analyzed in order to test the transforms between positive and negative interactions during the plant growth and the density changes. The composite De Wit instead of the experimental design was used to determine the length and width of leaf, root length, biomass and the root/shoot ratio of Festuca elta Keng and Lolium perenne L. at growth prophase, metaphase, anaphase, and the under different planting densities(low(32 plants/pot), middle(64 plants/pot), high(128 plants/pot)). In single population, leaf width was restricted by density in each period. Leaf length, root length, dry weight and root/shoot ratio were not controlled by density effect strictly at the prophase and metaphase. Whereas, the value decreased with the density raised at the anaphase. In mixed population, the morphology and in the mix-population were higher than those in single population at the anaphase period. This appeared earlier in greater planting density. Moreover, the decreasing biomass that resulted from density was lower than that in a single population. Mixture produced a significant decrease in the ratio of root to shoot compared with mono. The presence of the positive and negative interactions between F. elta Keng and L. perenne L. depended on the life history and the planting density. Particularly mixture improved the managed ecosystem stability in the anaphase. The higher density showed the negative effect, which decreased the positive plant interactions, but it would present earlier. The ratio of root to shoot decreased statistically significantly. It indictated that the plants showed potential niche and niche differentiation. Thus, mixture showed positive interactions under the same density, whereas the negative interaction was observed while the density increased. Mixture artificial grassland ecosystem was steadier than the single one.