Acidic polysaccharides, valued for their outstanding bioactivity and physicochemical properties, represent a promising strategy for metabolic disease intervention. In this study, three acidic polysaccharide fractions (BRP-1, BRP-2, and BRP-3) were isolated from Brassica rapa L. using membrane filtration and ion-exchange chromatography. BRP-3, notable for its high galacturonic acid content (76.64%), was further purified to yield the homogeneous fraction BRP-3-1 (Mw = 22.3 kDa). Combining GC-MS, FTIR, and NMR analyses, we report for the first time the detailed structure of BRP-3-1—a heteropolysaccharide composed of rhamnose (1.687%), galacturonic acid (75.584%), galactose (14.452%), and arabinose (8.277%)—with a backbone composed with T-α-L-Araf-(1 → 5)-α-L- Araf -(1 → 4)-α-D-GalpA-(1 → 4)-α-D-2-O- GalpA Me-(1 → 4)-α-D-GalpA-(1 → 4)-α-D-GalpA-(1 → 3)-Galp-(1 → 4)-α-D-GalpA, and T-Rhap, T-Galp as well as T-GalpA for branched chain and terminals. In HepG2 insulin-resistant cells, BRP-3-1 demonstrated potent dual regulation of glucose and lipid metabolism—enhancing glucose consumption, lowering total cholesterol, and significantly reducing triglyceride levels in the high-dose group (800 μg/mL), outperforming BRP-2. This work systematically defines the structure of a highly bioactive acidic polysaccharide from B. rapa L. and confirms its metabolic regulatory effects, offering a strong scientific foundation for its application in functional foods and as an adjuvant therapeutic for metabolic disorders.