Abstract: Objective: To investigate whether spleen deficiency state promotes the progression of Lewis lung carcinoma by regulating the gut microbiota. Methods: A spleen deficiency model was established in C57BL/6 mice using the water environment small platform method, followed by subcutaneous transplantation of Lewis lung carcinoma cells. The mice were divided into a blank control group (CK), a spleen deficiency model group (Model), a blank control with tumor transplantation group (KZ), and a spleen deficiency with tumor transplantation group (MZ). General signs, metabolic function, and behavioral performance of the mice were assessed. The final tumor volume and mass were measured. Histopathology was observed via HE staining. Serum levels of GDF-15, IL-6, and TNF-α were detected by ELISA. Splenic CD3?, CD4?, and CD8? T lymphocyte subsets were analyzed by flow cytometry. The gut microbiota structure was profiled using 16S rDNA sequencing. Results: Compared with the CK group, mice in the Model group exhibited decreased body weight, reduced grip strength and open field test indicators, and a lower urinary D-xylose excretion rate (P<0.05). Compared with the KZ group, mice in the MZ group showed a significant increase in transplanted tumor volume (P<0.01), elevated serum levels of GDF-15, IL-6, and TNF-α, a decreased splenic CD4?/CD8? ratio, reduced gut microbiota diversity, and a decline in the abundance of beneficial bacteria. Conclusion: Spleen deficiency state may promote the growth of Lewis lung cancer by affecting the gut microbiota and modulating the tumor immune-inflammatory response.