Fructans play vital roles in abiotic stress tolerance in plants. In this study, we isolated the sucrose:6-fructosyltransferase gene, which is involved in the synthesis of fructans, from Leymus mollis by rapid amplification of cDNA ends. The Lm-6-SFT gene was introduced into Arabidopsis thaliana cv. Columbia by Agrobacterium-mediated transformation. The transgenic plants were evaluated under salt stress conditions. The results showed that the expression of Lm-6-SFT was significantly induced by light, abscisic acid (ABA), salicylic acid (SA), and salt treatment in L. mollis plants. Overexpression of Lm-6-SFT in Arabidopsis promoted seed germination and primary root growth during the early vegetative growth stage under salt stress. We also found that the transgenic plants expressing Lm-6-SFT had increased proline and fructan levels. β-Glucuronidase staining and promoter analysis indicated that the promoter of Lm-6-SFT was regulated by light, ABA, and salt stress. Quantitative PCR suggested that overexpression of Lm-6-SFT could improve salt tolerance by interacting with the expression of some salt stress tolerance genes. Thus, we demonstrated that the Lm-6-SFT gene is a candidate gene that potentially confers salt stress tolerance to plants. Our study will aid the elucidation of the regulatory mechanism of 6-SFT genes in herb plants.