Abstract: Solid-state fermentation was conducted using Cordyceps militaris and Astragalus membranaceus residue to investigate the yield of main bioactive substances and their antioxidant effects in fermentation products. The yield of total polysaccharides, flavonoids, terpenoids, saponins and phenolics were measured respectively, and the total phenolic content of the fermentation product was extracted and optimized using response surface methodology. It was found that the total phenolic yield per unit weight in fermented products was significantly higher than that in Astragalus membranaceus residue. The optimal extraction conditions for fermented phenolics was: Ultrasonic temperature 80 ℃, duration 58 min, ethanol concentration 38.7%, pH5.4, achieving a total phenolic yield of 7.243±0.19 mg/g. Antioxidant experiments demonstrated that at 2.5 mg/mL, the maximum DPPH radical scavenging rate of total phenolics of the fermentation product was 94.94%, while that of Astragalus membranaceus residue was 64.56%. For ABTS⁺ radical scavenging, the maximum rate of total phenolics of the fermentation product was 89.51% at 1 mg/mL, while the maximum rate of total phenolics of Astragalus membranaceus residue was 59.54% at 2.5 mg/mL. Moreover, the total reducing power of total phenolics of the fermentation product was 2.046 at a concentration of 10 mg/mL, which was significantly higher than that in Astragalus membranaceus residue at 1.032 (P<0.001). In H₂O₂-induced PC12 oxidative stress models, fermented phenolics (10~10000 μg/mL) significantly enhanced cell viability compared to Astragalus membranaceus residue phenolics. Further analysis showed that the total phenolics of fermentation product led to a decrease in the level of MDA and an increase in SOD activity. These results indicate that solid-state fermentation significantly improves both phenolic yield and antioxidant capacity compared to unfermented Astragalus membranaceus residue. This study provides theoretical foundation for synergistic utilization of edible and medicinal fungi and Chinese herbal residues.