Abstract: In order to evaluate the effect of ultra-micro crushing on polyphenols, antioxidant activity and starch digestive enzyme activity of barley bran powder in vitro. A kind of barley bran coarse powder (335.94 μm) and two kinds of micro powder (micro powder A (72.52 μm) and micro powder B (22.69 μm) were prepared by hand-held omnipotent powder beating. The content and composition of polyphenols, flavonoids and their antioxidant properities, and starch digestive enzyme activities via Folin-ciocalteu and HPLC-MS methods were studied. The results showed that the contents of polyphenols, flavonoids and total phenols of micro powders were significantly higher than those in the crude powder (P < 0.05). Compared with coarse powder, the free phenols and flavonoids of micro powder B increased by 13.66% and 86.74%, and the bound phenols and flavonoids increased by 42.93% and 35.93%, respectively. Nineteen kinds of phenolic acids were detected in barley bran powder, of which ferulic acid and veratrum acid were the main free phenols, ferulic acid and benzoic acid were the main bound phenols. The total amount of phenolic acid in coarse powder, micro powder A and micro powder B was 57.23, 63.66 and 72.34 μg/g, respectively. Antioxidant activities of the two kinds of micro powders were significantly enhanced after ultra-micro crushing treatment (P<0.05). Compared with the coarse powder, the free DPPH · free radical scavenging ability, FRAP iron reduction capacity and ABTS+· free radical scavenging ability of micro powder B increased by 1.58%, 12.54% and 16.11%, respectively, while the bound DPPH· free radical scavenging ability, FRAP iron reducing ability and ABTS+·free radical scavenging ability was increased by 74.97%, 58.36% and 108.72% respectively. The antioxidant activities of micro powder B bound phenol were the most obvious. The particle size of barley bran power had negative contribution to its inhibition rate to α-glucosidase and α-amylase. Compared with coarse powder, the inhibition rate of free phenol on α-glucosidase was increased by 50.03% and 71.64%, and the inhibition rate of bound phenol on α - amylase was increased by 66.37% and 90.94%, respectively. The polyphenols and antioxidant activities of the powder were related to the activity of starch digestive enzyme. The results showed that the content of polyphenols, antioxidant activity and starch digestibility of barley bran could be improved by ultra-fine grinding to a certain extent, which could be used as an effective pretreatment and processing method of barley bran food.