In search of natural and safe compounds with immunomodulatory effects, this study identified a glucan with a molecular weight (Mw) of 1.2 × 10⁷ Da, named MSP-1-1, which was extracted and purified from Morchella sextelata via water extraction, alcohol precipitation, and column chromatography. Based on comprehensive characterization using HPAEC, SEC-MALLS-RI, FT-IR, GC-MS, NMR, and SEM, a structural model for MSP-1-1 is proposed. The model depicts a glucan with a backbone predominantly composed of →4)-α-D-Glcp-(1→ linkages, featuring occasional →4,6)-α-D-Glcp-(1→ residues that serve as branch points. The branches are identified as single α-D-Glcp-(1→ units attached at the O-6 position of these branching residues. In vivo experiments revealed that MSP-1-1 restored cyclophosphamide-induced abnormalities in immune organ indices, histology, and peripheral blood parameters. Additionally, MSP-1-1 significantly enhanced macrophage phagocytosis, splenic lymphocyte proliferation, and the proportions of CD3⁺CD4⁺ and CD3⁺CD8⁺T cells, while increasing the CD3⁺CD4⁺/CD3⁺CD8⁺ ratio. It also elevated concentrations of IgA and IgM in both serum and thymus, indicating immunomodulatory activity. In summary, this research elucidated the structural characteristics and immunomodulatory activity of MSP-1-1, providing insights into the bioactivity of M. sextelata glucan and a basis for further exploring its potential functional applications.