Abstract: Indoleamine 2,3-dioxygenase 1 (IDO1) is the rate-limiting enzyme in the degradation of tryptophan to kynurenine. IDO1 is highly expressed in some tumor tissues. IDO1 can deplete tryptophan in tumor microenvironment, inhibit T cell function, and mediate the immune escape of tumor cells. Thus, IDO1 is considered a potential target of tumor immunotherapy. Currently, there are several IDO1 inhibitors in clinical research studies. The mechanism of IDO1-mediated tumor immune escape and the structure of IDO1 inhibitors are summarized in this review.