Brassica rapa L. is an economic crop unique to plateau regions; its polysaccharides possess antioxidant, anti-fatigue, and anti-hypoxic properties. To investigate the structure of B. rapa L. polysaccharides and assess their antioxidant activity, crude polysaccharides were extracted using the water extraction and ethanol precipitation method, and BRP-5 was separated and purified. The composition, morphology, and structure of BRP-5 were analyzed using high-performance gel permeation chromatography, ion chromatography, scanning electron microscopy, scanning tunneling microscopy, infrared spectroscopy, and synchronous thermal analysis. The in vitro antioxidant activity of BRP-5 was evaluated by measuring its ability to scavenge superoxide, hydroxyl, and DPPH radicals. The results indicated that BRP-5 is an acidic polysaccharide with galacturonic acid as the primary monosaccharide and has a weight-average molecular weight of 16 670 Da. Cracks and fissures were identified on its surface, with the ten-point height (Sz), root-mean-square height (Sq), and roughness average height (Sa) being 24.40, 2.84, and 2.01 nm, respectively. Further analysis revealed that BRP-5 may contain pyranose rings and exhibited an amorphous structure. BRP-5 also exhibited certain antioxidant properties: scavenging rates of DPPH and hydroxyl radicals showed dose dependence within the range of 0.1~1.5 mg•mL-1, with the 50% inhibiting concentration (IC50) values being 0.96 mg•mL-1 and <0.1 mg•mL-1, respectively; the scavenging rate of superoxide anion radicals was <50%. The findings of this study provide novel insights into the structure of B. rapa L. polysaccharides and will aid in the development of related antioxidant products.