Abstract: This study aimed to systematically compare the in vitro antioxidant activities of aqueous extracts and polysaccharides isolated from the bulbs and flowers of Fritillaria thunbergii Miq., and to investigate the composition of floral polysaccharides as well as their in vivo antioxidant effects and preliminary mechanisms in the model organism Caenorhabditis elegans, thereby providing experimental evidence for the utilization of F. thunbergii Miq. flowers. Bulb decoctions (FTD), bulb polysaccharides (FTP), flower decoction (FTFD), and floral polysaccharides (FTFP) from F. thunbergii Miq. were prepared using water extraction. Their in vitro antioxidant activities were evaluated using 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2’-azino-bis (3-ethylbenzthiazoline-6-sulphonate) (ABTS) radical scavenging assays, as well as a ferric ion reducing capacity (ferric reducing antioxidant power, FRAP) assay. Furthermore, the C. elegans model was used to assess the antioxidant effects of the most active component screened. The regulatory effects on the expression of oxidative stress-related genes were explored by analyzing the fluorescence intensities of transgenic C. elegans CF1553 (sod-3p::GFP) and CL2166 (gst-4p::GFP). The results revealed that the flower-derived extracts (FTFD and FTFP) exhibited significantly stronger antioxidant capacities than the bulb-derived extracts (FTD and FTP). Among these, FTFP (1 mg/mL) showed the strongest ferric reducing ability (0.97±0.011 mmol/L), along with high scavenging rates against DPPH radicals (76.58%±0.68%) and ABTS cation radicals (58.63%±8.58%). In vivo experiments showed that C. elegans treated with FTFP (0.04~4 mg/mL) exhibited significantly enhanced resistance to heat and oxidative stress and up-regulated the expression of the antioxidant genes sod-3 and gst-4 in a concentration-dependent manner. Composition analysis revealed that FTFP was an acidic heteropolysaccharide primarily composed of galacturonic acid (GalA, 70.02%), and contained rhamnose, glucose, galactose, xylose, and arabinose. High-performance gel permeation chromatography (HPGPC) showed that it primarily contained two components with average molecular weights of 363.05 and 26.04 kDa, respectively. In conclusion, FTFP is the primary active component responsible for its antioxidant effects, as it effectively scavenges free radicals as well as enhances the resistance of organism to oxidative stress by upregulating sod-3 and gst-4 expressions. This study provides an important theoretical basis for the development and use of F. thunbergii flower resources.