Abstract: Indoleamine 2, 3-dioxygenase-1 (IDO1) is a key rate-limiting metabolic enzyme which catalyzes the degradation of the essential amino acid tryptophan along the kynurenine pathway. It participates in tumor immune escape and has been found to be overexpressed in various cancer types, which is closely associated with the malignant transformation of tumors and the poor prognosis of cancer patients. IDO1 has emerged as a promising target for immunotherapeutic agents against tumors. To date, a number of structurally diversified IDO1 inhibitors have been reported, with quite a few having entered clinical trials. This paper summarizes the recent progress in the study of the mechanisms of action of IDO1 in tumor immune escape and its related drugs, so as to provide new ideas for tumor immunotherapy.