Based on a greenhouse simulation experiment, this paper studied the dynamics of the above- and below-ground biomass of Leymus chinensis populations with four planting densities (120, 240, 360 and 480 plants x m(-2)). The results showed that the above- and below-ground biomass of L. chinensis populations had an increasing trend with the growth, and increased with increasing planting density. However, the relative growth rates (RGRs) of the above- and below-ground biomass did not show significant difference among the L. chinensis populations. The ratio of root to shoot (RRS) of the L. chinensis populations increased with their growth, but did not show significant difference among the planting densities, implying that environmental factors were the controlling factors for RRS. There existed significant power function relationships between the above- and below-ground biomass of L. chinensis individuals and populations, but the power function indices and coefficients varied with the planting density. It was suggested that the difference of microenvironment resulted from resources competition could induce the changes of L. chinensis RRS, and the latter could be used as a reference to quantitatively study the allocation of photosynthetic products.