Abstract: Objective: To explore the protective effect of lactulose regulating the nicotinamide adenine dinucleo-tide phosphate oxidase 4 (NOX4)/nucleotide binding domain like receptor protein 3 (NLRP3) pathway on rats with rheumatoid arthritis (RA).Methods: Fifty rats were randomly divided into control group, RA group, lactu-lose group(0.25 g/kg), lactulose+GKT137831(NOX4 inhibitor)(10 mg/kg)group, methotrexate group(0.35 mg/kg), with 10 rats in each group.Except the control group, RA rat model was established by intracutaneous injec-tion of collagen lotion into the tail root in other groups.After modeling, each group was given corresponding drug treatment by intragastric administration.Arthritis score of the rats was performed the next day after gavage.The proportion of intestinal flora in feces of the rats was detected by real-time fluorescence quantitative poly-merase chain reaction (RT-qPCR).The levels of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) in se-rum of the rats were detected by enzyme-linked immunosorbent assay(ELISA).Hematoxylin-eosin(HE)staining was employed to observe the pathological changes of ankle joint.NOX4 and NLRP3 protein expressions were detected by western blotting.Results: Compared with the control group, pathological phenomena such as inflammatory infiltration, small amount of fibrin deposi-tion and synovial hemorrhage were observed in rats in RA group.The proportions of Bacteroides, Bifidobacterium and Lactobacillus in feces decreased obviously, while the proportion of Actinomycetes, the average arthritis score, the levels of TNF-α and IL-6 in serum, and the expressions of NOX4 and NLRP3 proteins in joint tissue in-creased obviously (all P< 0.05); compared with the RA group, the pathological phenomena of lactulose group and methotrexate group were improved, the proportions of Bacteroides, Bifidobacterium and Lactobacillus in fe-ces increased obviously, the proportion of Actinomycetes, the average arthritis score, the levels of TNF-α and IL-6 in serum, the expressions of NOX4 and NLRP3 proteins in joint tissue decreased obviously (all P< 0.05), and there was no obvious difference between the two groups(P> 0.05); compared with lactulose group, the pathologi-cal phenomenon of lactulose+GKT137831 group was further improved, the proportions of Bacteroides, Bifidobacterium and Lactobacillus in feces increased obviously, the proportion of Actinomycetes, the average arthritis score, the levels of TNF-α and IL-6 in serum, and the expressions of NOX4 and NLRP3 proteins in joint tissue de-creased obviously (all P< 0.05).Conclusion: Lactulose can reduce the inflammatory reaction in RA rats, its mechanism may be related to the inhibition of NOX4/NLRP3 pathway.