BACKGROUND: FOLFOX is the recommended chemotherapy regimen for colorectal cancer (CRC), but its response rate remains low. Our previous studies have established a close relationship between gut microbiota and the anti-CRC effect of FOLFOX, though the underlying mechanisms remain unclear. Diet has been confirmed as a key factor influencing gut microbiota, and high-salt diets, representative of western dietary habits, has been shown to affect gut microbiota, immune function, and the risk of developing CRC. However, the impact of high-salt diets on the anti-CRC efficacy of FOLFOX remains unstudied. Therefore, we aimed to investigate the effect and mechanism of high-salt diets on the anti-CRC effect of FOLFOX. METHODS: We performed 16 S rRNA sequencing and T500 targeted metabolomics analysis on fecal samples from CRC patients and healthy adults. A CRC orthotopic xenograft mouse model was used to study the effect of a high-salt diet on FOLFOX's anti-CRC efficacy. 16 S rRNA sequencing and non-targeted metabolomics were conducted on mouse fecal samples. Flow cytometry was used to assess immune cell infiltration in tumor and paracancerous tissues. A mouse macrophage conditioned medium system, with tryptophan metabolites, was employed to annotate the functional metabolites, followed by in vivo verification using the orthotopic xenograft mouse model. RESULTS: The structure and metabolic profiles of gut microbiota are significantly different between 9 healthy adults and 6 CRC patients. A high-salt diet significantly reduced the efficacy of FOLFOX in mice, with notable changes in gut microbiota and related metabolites. Correlation analysis revealed a significant relationship between gut microbiota, tryptophan metabolites and FOLFOX efficacy. Flow cytometry indicated that a high-salt diet altered macrophage infiltration (CD45 CONCLUSION: A high-salt diet reduces the anti-CRC efficacy of FOLFOX through gut bacterial tryptophan metabolism mediated macrophage immunomodulation.