Abstract: To investigate the effects of particle size and phenolics bounded-insoluble dietary fiber (IB) of white kidney bean skin (WS) on in vitro fermentation profiles, the IB was extracted enzymatically, modified by ordinary and superfine grinding, followed by the preparation of different particle sizes. Their microscopic morphology was observed by scanning electron microscopy and laser confocal scanning microscopy. In vitro fermentation experiments and 16S rRNA sequencing were used to investigate the fermentation properties of the two sets of samples and their effects on the abundance of gut microbiota composition. The results showed that the bound phenolics were distributed more equally on the surface of the smaller particles. The slowed fermentation rate and short-chain fatty acid production of WS and IB increased with decreasing particle size. The smaller particle size promoted the Bacteroides and demoted the Lachnospira more significantly. The results suggested that IB could promote the beneficial bacteria such as Bacteroides, Bifidobacterium and Ruminococcaceae ruminococcus, and regulate the structure composition of the gut microbiota. The probiotic effect of IB was more significant than that of WS with the same particle size. This study provides the school of thought on the prebiotic effect of dietary fiber, guides on the modification of dietary fiber by superfine grinding, and suggests the comprehensive utilization of white kidney bean skin.