Abstract: PD-1 antibody immunotherapy has been used as a first-line treatment against various malignancies, but resistance to this treatment limits its efficacy. For instance, myeloid derived suppressor cells, myeloid derived suppressor cells induce resistance to PD-1 antibody in a tumor microenvironment. A few combination regimens of an IDO inhibitor plus PD-1 antibody are currently subjected to ongoing clinical trials in the US, and preliminary results have shown that this inhibitor can reverse the resistance of malignancies to PD-1 antibody. This study reviewed the research progress on the resistance mechanism of malignancies to PD-1 antibody and revealed that IDO inhibitor regulates MDSCs to reverse the resistance to PD-1 antibody. This study also described the clinical efficacy of this inhibitor plus PD-1 antibody.