Arbuscular mycorrhizal fungi (AMF) could improve the tolerance of plants under saline stress. However, the degree of this improvement might differ relative to salt stress intensity. We conducted an experiment to explore the ability of AMF to colonize Leymus chinensis roots and evaluate the effect of AMF inoculation on the growth of plants under four NaCl concentrations (0, 1, 2, and 3 g NaCl/kg soil). The mechanism behind the saline tolerance of mycorrhizal L. chinensis plants was also discussed in terms of their root:shoot ratio and ionic content. The inoculants used in this study were Glomus mosseae and G. geosporum, two dominant species found in highly saline-alkaline natural grasslands. Sixteen pots of inoculated L. chinensis seedlings were randomly subjected to one of four NaCl treatments, as were sixteen pots of non-inoculated plants of the same species. The plants were harvested and measured after 75 d of growth. The results show that under high saline stress, the AMF significantly decreased the strength of the salinity response and increased plant biomass. The mycorrhizal growth response under high saline stress was significantly positive, which indicated that the AMF-plant symbiotic relationship is important to plants under salt stress. However, the colonization rate and infection intensity of AMF in L. chinensis roots both decreased as salt stress increased. The ability of AMF to improve plant growth might decrease, and one-sided or mutually harmful negative effects might occur when salt stress becomes worse, because of the increasing intensity of the competition between AMF and plants. The AMF significantly increased the root:shoot ratio in each NaCl treatment. More photosynthetic product was allocated to the roots of mycorrhizal plants than those of non-mycorrhizal plants, which increased both the water and nutrient absorption, and transportation capacity of the AMF-plant symbiont. The re-allocation of resources between roots and shoots might be one of the primary strategies used by mycorrhizal plants to increase their fitness under harsh environmental conditions. The higher N and P concentrations observed in mycorrhizal plants also support the idea that AMF help L. chinensis absorb and utilize more nutrients under saline stress. The concentrations of Na⁺ and Cl^- were both lower in mycorrhizal plants than in non-mycorrhizal plants, decreasing the ionic content of inoculated plants. Ca²⁺ and P/Na⁺ concentrations were higher in mycorrhizal plants under low saline stress, while the K⁺ and K⁺/Na⁺ concentrations were higher in mycorrhizal plants under high saline stress, indicating that the mycorrhiza could reduce the physiological drought response of plants under saline stress by adjusting their osmotic potential via absorbing other ions and/or increasing ionic ratios such as K⁺/Na⁺. The results also showed that the saline tolerance of mycorrhizal plants was increased by the mycorrhizal P response under lower saline stress, and by the mycorrhizal K response under high saline stress. This study has improved the understanding of the mechanism underlying the improved saline tolerance of mycorrhizal plants, and provides some guidelines for using AMF technology to restore saline-degraded grasslands.