Abstract: Although immune checkpoint inhibitors (ICIs) have great breakthrough in cancer treatment in recent years, most patients have not benefited from it on account of immune microenvironment. Studies have shown that tryptophan metabolism is not only involved in the formation of tumor immunosuppressive microenvironment but also plays an important role in the therapeutic application of ICIs. At present, inhibiting the kynurenine pathway of tryptophan metabolism is now in various stages of clinical trials, while the other two metabolic pathways, 5-HT and the indole pathway, also have aroused wide concern. This article reviews the latest developments in this field.