This study aimed to characterize the structural features of a novel water-soluble polysaccharide (AOHP) extracted from Alpinia officinarum Hance and to verify its regulating effect on mouse immunity. Cellulose DEAE-52 and Sephadex G-100 columns were used to obtain purified AOHP. Techniques including NMR, methylation, monosaccharide composition, FT-IR, and molecular weight determination were applied to investigate the physicochemical properties and structural characterization of AOHP. Then, the influence of AOHP on mice was studied. After oral administration of AOHP, organ indexes, serum biochemistry indexes, and cytokines in the spleens of the mice were analysed. The results showed that AOHP was composed of T-α-D-Glcp, (1,4)-α-D-Glcp and (1,4,6)-α-D-Glcp with a number-average molecular weight of 26.0 kDa and a weight-average molecular weight of 52.8 kDa. Additionally, the innate immune statuses of the mice were improved by treatment with AOHP, while no obvious damage was identified. To conclude, the immunomodulatory activity and biological safety make AOHP a viable candidate as an ingredient for healthcare drugs.