Inulin has potential benefits for alleviating intestinal stress syndrome, constipation, and immunomodulation. However, its effects on cat gastrointestinal tract remain unexplored. Eight healthy adult British short-haired cat were administered 50 mg/kg/d inulin with a basal diet for 21 days, while fecal samples were collected to measure indole and 3-methylindole levels, immune index detection, and fecal microbial diversity on days 0, 7, 14, and 21. The results showed that adding inulin to the diet of cat could cause the increase of sIgA on day 14 (P < 0.05) and enhance their immune performance. In addition, it will also affect the fecal microbiota of the cat. Collinsella abundance was significantly increased, which could indulge ursodeoxycholic acid production. Feeding inulin had no significant effect on the levels of indole and 3-methylindole (P > 0.05). The above results showed that inulin supplementation in cat diet could improve cat health by enhancing immunity and increasing intestinal beneficial flora.