[Objective] This study was to investigate the effect of a fermented feed on colonic fermentation and microflora of finishing pigs. [Methods] Capillary column gas chromatography was used to measure volatile fatty acid (VFA) concentration. Illumina MiSeq platform was used to detect community structure and diversity of the microflora of colonic mucosa and digesta. [Results] Fermented feed had no significant effect on microbial diversity of colonic mucosa and digesta (P>0.05), while increased the relative abundance of Weissella and Faecalibacterium in colonic mucosa (P<0.05). The relative abundance of Weissella and Subdoligranulum was higher in colonic digesta (P<0.05) compared with the control group. No significant difference was observed in pH, lactate, acetate, propionate, isobutyrate, valerate, isovalerate and total VFA concentration between the two groups (P>0.05), whereas butyrate concentration was greater in the treatment (P<0.05). [Conclusion] Fermented feed could affect the composition of colonic microflora and promote the formation of colonic butyric acid in finishing pigs, which could improve their intestinal health.