Longan (Dimocarpus longan Lour.) aril is a traditional medicinal and edible substance with anti-aging properties. Prolonged steaming, a classical processing method, can enhance its tonic efficacy and mitigate its dryness. However, the structural evolution of polysaccharides, the principal bioactive constituents, remains poorly understood during this process. To systematically investigate the structural characteristics and anti-aging activity of longan aril polysaccharides before and after prolonged steaming, polysaccharides were extracted from raw and prolonged-steamed longan aril, respectively, and subjected to structural characterization and anti-aging activity evaluation. The results showed that after prolonged steaming, the total sugar content of longan polysaccharides decreased from 358.24 mg·g-1 to 332.26 mg·g-1, whereas the contents of total phenolics and total flavonoids increased from 91.45 mg·g-1 and 138.73 mg·g-1 to 154.62 mg·g-1 and 166.63 mg·g-1, respectively. The molecular weight distribution shifted toward lower molecular weights. The monosaccharide composition analysis revealed that the molar ratio of glucose increased from 70.50% to 97.33%, with concomitant reductions in all other monosaccharides, suggesting the degradation of glucogalactan, rhamnogalacturonan, and homogalacturonan structural domains, and indicating that glucan domains became predominant in the steamed polysaccharides. Attenuated total reflection Fourier transform infrared spectroscopy showed a decrease in the intensity and narrowing of the O-H stretching peak after steaming, suggesting weaker intermolecular hydrogen bonding and a looser chain conformation. X-ray diffraction analysis indicated increased short-range order, and surface morphology observation revealed thinner lamellae, increased pores, and curled edges. Compared with the un-steamed counterpart, the steamed longan polysaccharides exhibited significantly enhanced scavenging activities against DPPH, ABTS, and hydroxyl radicals. In the Caenorhabditis elegans model, the steamed polysaccharides significantly prolonged the mean lifespan, reduced reactive oxygen species and lipofuscin accumulation, enhanced tolerance to oxidative and thermal stress, improved locomotor capacity, and did not affect reproductive ability, demonstrating favorable antioxidant, anti-aging, and safety profiles. This study is the first to systematically reveal the effects of prolonged steaming on the structure and anti-aging activity of longan polysaccharides, providing polysaccharide-level experimental evidence for the scientific basis of the traditional “enhanced efficacy and reduced dryness” processing principle. It also supplies data supporting the study of structure-activity relationships of polysaccharides from tonic medicinal and edible substances during steaming.