White H. marmoreus is widely used as an ingredient of traditional Chinese dishes. In this study, a β-glucan noted as WHP was isolated from alkaline extract of white H. marmoreus. The structure of WHP was investigated by FT-IR, NMR, methylation and partial acid hydrolysis. The results showed that the main chain of WHP was composed of (1→3)-β-D-Glcp, with an average of three to four residues of O-6 linked with 1-β-D-Glcp. Moreover, a nanoderivative (AgNPs) of WHP was prepared. The most AgNPs were synthesized when the concentration ratio of AgNO3 to WHP was 0.25. Immunological activity tests indicated that WHP had superior cell proliferation activity and its maximum proliferation index was 1.62. AgNPs could also stimulate the proliferation of macrophage RAW264.7. WHPS could better increase the phagocytosis, immunological activity of macrophage RAW264.7. The difference in immunological activity between WHP, AgNPs and WHPS evidently resulted from their structural distinction.