To examine the effect of walnut green husk powder on fecal microflora and serum immune indices in fattening pigs, sixty Duroc×Landrace×Yorkshire castrated male finishing pigs(with body mass of 80 kg) were randomly divided into negative control group(NC) and walnut green husk powder group(WP), with three replicates in each group and ten pigs in each replicate. The NC group was fed with basic diet, and the WP group was fed with basic diet supplemented with 0.1%(mass fraction) walnut green husk powder, with a pre-experimental period of 10 days, and an experimental period of 40 days(the pigs were ad libitum fed every day during the trial). Fresh feces and blood samples were collected from these two groups at the end of the experimental period, and the diversity of the species, the change pattern of community structure and the serum immunity indexes were analyzed. The results showed that, there was no significant difference in bacterial diversity and community structure between these two groups(P>0.05). Firmicutes, Proteobacteria and Actinobacteria had higher percentages in both groups, and Lactobacillus, Clostridium, Shigella, SMB53, Streptococcus and Turicibacter were the dominant genera. Compared with NC group, the relative bundance of Lactobacillus extremely increased(P<0.01), and that of Turicibacter was extremely reduced in WP group(P<0.01). In non-pathological conditions, mass concentration of serum IgG was extremely increased(P<0.01), while those of IgA and IgM were significantly increased in WP group(P<0.05). In summary, the supplementation of walnut green husk powder to diets could regulate the abundance of fecal microflora, improve intestinal health, and enhance the immune function of fattening pigs.