Abstract: This paper used vegetable soybeans which were not qualified for export as raw materials to increase total flavonoid yield through Bacillus natto fermentation, thus providing theoretical support for increasing the added value of vegetable soybeans. Bacillus natto was used to ferment vegetable soybeans, and the total flavonoid content in the fermented product was used as criteria to determine the optimal fermentation conditions. The extracted total flavonoids were purified using D101 macroporous resin. The content of total flavonoids was determined by rutin colorimetric method. The in vivo experiments were divided into obesity preventive and alleviating tests using C57BJ/6L mice. Mice from the preventive experiment were fed high-fat diet while administered flavonoid extracts (50 mg/kg·d) through gavage; in alleviating groups, mice were administered total flavonoid extracts at low, medium and high doses (20, 50, and 100 mg/kg·d, respectively) through gavage after establishing obese mouse model with high-fat diet. The hypolipidemic effect of total flavonoids was evaluated through parameters such as mouse body weight, blood lipids and pro-inflammatory factors. Results showed that the optimal conditions for production of total flavonoids via fermentation were as follows: Bacillus natto was inoculated at 1.15 × 10¹¹ CFU/100 g vegetable soybeans, which was subjected to fermentation at 37 ℃ for 24 h and post ripeness for another 12 h. The yield of total flavonoids was 4.30 mg/g vegetable soybean, with the content of flavonoids in the crude extract at 22.1%. In the prevention experiment, there were no significant differences in both the weight and Lee's index of the high-fat diet mice after gavage with total flavonoids compared with those of the normal control group (P >0.05). In the alleviating experiment, the serum triglyceride (TG) content of the mice treated with high, medium, and low-dose flavonoid extracts were 313.10 ± 10.12, 310.39 ± 31.76 and 310.10 ± 10.20 nmol/L, respectively, with no significant difference compared with that of the normal control group (330.80 ± 34.36 nmol/L) (P>0.05). After intragastric intervention with flavonoids, serum total cholesterol (TC), interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) levels in mice showed a significant decrease compared with those of the higher fat control group (P<0.05). In summary, fermentation using Bacillus natto helped to increase the total flavonoid content in vegetable soybeans. The flavonoid extract showed the effect of both preventing and alleviating mouse hyperlipidemia.