The chemical structure of a water-soluble pectic polysaccharide (sterculia APS) isolated from boat-fruited sterculia seeds (Semen Sterculiae Lychnophorae) was elucidated using partial acid hydrolysis, methylation analysis and 1D/2D nuclear magnetic resonance (NMR) spectroscopy. The results of partial acid hydrolysis revealed that sterculia APS contained galacturonic acids and rhamnose units in the backbone, and arabinose, galactose and xylose residues in the branched chains. Combined the methylation analysis results with NMR spectroscopy, a possible structure of APS was proposed as follow: the backbone consisted of the homogalacturonan (HG) named "smooth" regions by linear linkage of 1,4-α-D-GalpA, and the rhamnogalacturonan-I (RG-I) called "hairy" ones by the linkage of →4)-α-D-GalpA-(1→2)-α-L-Rhap-(1→partially substituted at O-4 of rhamnose units. The GalpA residues were partially methyl esterified and O-acetylated on C-2 and/or C-3. The side chains were mainly composed of araban and arabinogalactan by the linkages of T-, 1,3-, 1,5-L-Araf and T-, 1,4-, 1,6-, 1,3,6-, 1,3,4-D-Galp, attached to O-4 of the backbone rhamnose units.