Leymus chinensis and Stipa grandis are two of the main species of typical grasslands of Xilingol, Inner Mongolia, and are important forage grasses. An enclosed grassland, with L. chinensis and S. grandis as the dominant species, in Xilingol was selected as the research area, and moisture gradient control experiments (150, 300, 450, 600 mm) were carried out over two years by transplanting the original soil column, to simulate changes of local annual rainfall from drought to wet years, and the response of plant height and root architecture of L. chinensis and S. grandis populations were analyzed and compared under the different moisture gradients. The results showed that the response of L. chinensis populations was more sensitive to moisture gradients than that of S. grandis. With increasing moisture gradients, the plant height and root diameter of L. chinensis increased, whereas the root length and depth decreased significantly((P < 0.05). However, for S. grandis, the indicators of root architecture and plant height showed no correlation with the moisture gradient. Therefore, S. grandis could adapt to arid habitats more effectively than L. chinensis, and under drought conditions (simulated rainfall of 150 mm) the plant height of S. grandis populations reached a peak value and grew efficiently, whereas L. chinensis populations adopted a growth strategy of individual miniaturization, with thinner taproots and bifurcation to extend to the deep soil. Therefore, with climate change, a drying climate will result in increased S. grandis populations as the dominant species in the community, and L. chinensis populations will have a greater advantage in wetter climates.