Sintilimab (Sin) helps the body to restore the anti-tumor response of T lymphocytes. However, in clinical use, the treatment process is more complicated due to adverse effects and different dosing regimens. It is not clear whether prebiotics (PREB) have a potentiating effect on Sin for lung adenocarcinoma, and this study intends to investigate the inhibitory effect, safety and possible mechanism of Sin combined with PREB on lung adenocarcinoma from animal experiments.Lewis lung adenocarcinoma cells were inoculated into the right axilla of mice subcutaneously to prepare the Lewis lung cancer mouse model and treated in groups. The volume of transplanted tumors was measured, the histopathology of the liver and kidney of mice was observed by H&E staining, the levels of ALT, AST, UREA, CREA, WBC, RBC, and HGB in blood were analyzed biochemically; the ratio of T-cell subpopulations in blood, spleen, and bone marrow was detected by flow cytometry, the expression of PD-L1 in tumor tissue was detected by immunofluorescence staining, and 16S rRNA to analyze the diversity of fecal flora.Sin inhibited tumor growth and regulated immune cell homeostasis in lung adenocarcinoma mice, but liver and kidney histopathology showed different degrees of damage after Sin treatment, while the addition of PREB reduced liver and kidney damage in lung adenocarcinoma mice and promoted Sin's regulation of immune cells. In addition, the beneficial effects of Sin were associated with changes in intestinal flora diversity.The mechanism by which Sintilimab combined with prebiotics inhibits tumor volume and regulates immune cell subpopulation balance in lung adenocarcinoma mice may be related to gut microbes.Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.net.