Oat bran is a by-product of oat， which is an excellent source of phytochemicals such as dietary fiber and polyphenols. Therefore， the aim of this study was to evaluate the effects of different levels (1%， 3% and 5%) of oat bran on fecal microbiota composition， short-chain fatty acids and antioxidant capacity in an obese population， by simulating in vitro digestion and fermentation processes. The results showed that the antioxidant activity of oat bran was significantly increased after digestion(P<0.05) and the antioxidant activity was higher with increasing levels. The microbial results showed that the main dominant bacteria at the phylum level were Proteobacteria (29.01%-46.55%)， Actinobacteria (7.82%-16.18%)， Firmicutes (44.63%-51.98%) and Bacteroides (0.05%-2.58%). The microbial α-diversity decreased in the group with the addition of oat bran compared to the control group， as well as the number of Firmicutes and Actinobacteria， indicating that oat bran had an inhibitory effect. In addition， it was found that the main short chain fatty acids after oat bran fermentation were acetic acid， propionic acid and butyric acid. The total short chain fatty acid content in the FOB5 group was 161.21 μg/mL， significantly lower than other groups. In conclusion， oat bran has good antioxidant properties. And it reduces the content of short-chain fatty acids by regulating the abundance of intestinal microorganisms. It is promising to provide a theoretical basis for the weight loss and lipid reduction mechanism of oat bran.