Objective To improve the transformation of functional components in the Euryale ferox by microbial solid-state fermentation.Methods Euryale ferox is taken as the raw material to screen out the dominant strains, with the total flavonoid and total polyphenol content as indicators. Then, orthogonal experiments are conducted on the dominant bacteria to investigate the effects of fermentation temperature, fermentation time, material-liquid ratio, inoculation amount, and carbon source addition amount on the active components of the fermented Euryale ferox. The in vitro antioxidant activity of Euryale ferox is analyzed, with DPPH free radical scavenging rate, total antioxidant activity, and ABTS free radical scavenging rate as indicators.Results The dominant bacterium is Aspergillus niger H, and the optimal fermentation condition is as follows: fermentation time 5 days, fermentation temperature 35 ℃, material-liquid ratio of 1∶2 (g/mL), inoculation amount of 11%, and carbon source addition amount of 2%. Under these conditions, the total flavonoid content is 9.036 mg/g, which is 6.24 times higher than that of the unfermented one, and the total polyphenol content is 109.284 mg/g, which is 4.68 times higher than that of the unfermented one. The IC₅₀ values of DPPH and ABTS free radical scavenging rates of fermented Euryale ferox are 0.023 and 0.024 mg/mL, respectively. The reducing ability of vitamin C positive control to iron ions is basically the same as that after fermentation, and is higher than that before fermentation.Conclusion It is feasible to use microbial to ferment the raw materials of Euryale ferox, which improves the extraction content and antioxidant activity of its active ingredients, demonstrating an ideal application prospect.