A neutral α-glucan, named BP1, with a molecular mass of approximately 9.45 kDa, was isolated from Lobelia chinensis by hot-water extraction, a Q-Sepharose Fast Flow column and Superdex-75 column chromatography. Its chemical structure was characterized by monosaccharide analysis, methylation analysis and analysis of its FT-IR, high performance gel permeation chromatography (HPGPC) and 1D/2D-NMR spectra data. The backbone of BP1 consists of →α-D-Glcp→α-D-Glcp→(α-D-Glcp)x-α-D-Glcp-(α-D-Glcp)y→. The side chains were terminal α-D-Glcp1→ and α-D-Glcp1→ (α-D-Glcp)z→α-D-Glcp→α-D-Glcp→α-D-Glcp→ (x + y + z = 5), which are attached to the backbone at O-3 of α-D-Glcp. The results of the effect of BP1 on mouse macrophage cell line RAW 264.7 indicate that BP1 enhances the cell proliferation, phagocytosis, nitric oxide production and cytokine secretion in a dose-dependent manner. Because the inhibitor of Toll-like receptor 4 blocks the BP1-induced secretion of TNF-α and IL-6, we hypothesize that α-glucan BP1 activates TLR4, which mediates the above-mentioned immunomodulating effects.