Lymphocyte activation gene 3 (LAG-3) is a novel immune checkpoint molecule, which is mainly expressed in activated T cell and regulatory T cells (Tregs). Induced by T cell receptors (TCR) and cytokines, LAG-3 is activated by binding to ligands such as MHC II and fibrinogen like protein 1, which then upregulates function of Tregs, negatively regulates TCR signal transduction and affects cytokine secretion. All these regulations lead to dropped body immunity and the immune escape of pathogens. In recent years, LAG-3 has become a new potential target for tumor immunotherapy due to its multiple ligands, wide expression and significant efficacy. Hereby, we reviewed and summarized the structure, ligand and function of LAG-3, as well as its application in tumor immunotherapy and parasitic infection.