Abstract: Studies have found that a variety of tumors continue to activate PD-1(programmed cell death protein 1, PD-1)/PD-L1(programmed cell death-ligand 1)signaling pathway by up-regulating PD-L1 expression in tumor cells and microenvironment. The dysfunction of T cells leads to the occurrence of tumor immune escape. Several PD-1/PD-L1 monoclonal antibodies have been marketed to achieve significant clinical efficacy. However, because of the high production cost, the harsh conditions for storage and transportation, and the potential immunogenicity of monoclonal antibody, the seeking for PD-1/PD-L1 small molecule inhibitors has become a hot spot in the development of new drugs. In this paper, the biological mechanisms of PD-1/PD-L1 was introduced in detail. Based on the structural classification, the research progress of PD-1/PD-L1 small molecule inhibitors was reviewed, with a prospect of the development of small molecule inhibitors.