Objective: This study investigated the phenolic composition and in vitro gastrointestinal digestion and fermentation characteristics of buckwheat hulls (BH) fermented by Lactobacillus plantarum HH-LP56. Methods: The temporal changes in pH value， total phenolic content， and antioxidant activity during L. plantarum fermentation of BH were analyzed， and several representative fermented samples were selected to evaluate the phenolic compositions by HPLC. The bioaccessibility of BH phenolics were evaluated using in vitro gastrointestinal digestion model named Infogest 2.0， and the pH， gas and SCFA production were analyzed to characterize the fermentation properties of BH. Results: The fermented BH at 6 h (LP-6h) significantly increased the total phenolic content， which further increased in the fermented BH at 24 h (LP-24h). The total phenolic content reached the highest level (7.32 mg/g) at 48 hours of fermentation (LP-48h). Analysis of the composition and forms of individual phenolics in unfermented (Raw) and fermented BH revealed that the content of free gallic acid (ranging between 142.88 and 145.87 mg/kg) and protocatechuic acid (ranging between 103.30 and 113.61 mg/kg) in LP-6h， LP-24h， and LP-48h was significantly higher than that in the Raw group (P < 0.01)， as a result， the intestinal bioavailability of gallic acid and protocatechuic acid was significantly increased. Through the inoculation of 5 volunteers' fecal microbiota to simulate colonic fermentation， the results suggested that the gas production， three main SCFA and BCFA contents of Raw and fermented BH were similar. However， within three volunteers， the total SCFA content of LP-6h， LP-24h， and LP-48h was significantly higher than that of Raw BH after 48 hours fermentation (P < 0.01 or P < 0.05). Conclusion: L. plantarum fermentation can effectively improve the gastrointestinal digestion and fermentation characteristics of BH， providing a theoretical basis for the applications in the deep and value-added processing of BH.