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A novel polysaccharide DG2 was obtained from the Dendrobium officinale endophytic fungus Fusarium solani DO7. The structural characterization and immunological activity of DG2 were investigated. Results showed that DG2 with the molecular weight of 152.8 kDa was composed of Galp, Manp, Glcp and Araf at a molar ratio of 4.3:1.9:2.8: 1.0. The main linkage types of DG2 were (1 → )-β-D-Glc, (1 → 4)-β-D-Glc, (1 → 3,6)-β-D-Gal and (1 → 6)-β-D-Gal, which demonstrated DG2 was a novel galactoglucan and different from other fungal and botanic polysaccharide. Furthermore, DG2 did not contain deoxynivalenol, and showed no direct toxicity to human embryonic kidney cells and RAW 264.7 cells. Meanwhile, DG2 could increase the TNF-α, IL-6 and iNOs mRNA expression levels of RAW264.7 cells, and further promote the production of TNF-α, IL-6 and NO. These results suggested DG2 could enhance macrophage-mediated immune responses and have the potential to be an attractive functional food supplement candidate for the hypoimmunity therapies.