Interleukin-27 (IL-27) is a heterodimeric cytokine composed of p28 and Epstein-Barr virus-induced 3 (EBI3), which belongs to the IL-6/ IL-12 family group. IL-27 exerts its biological functions mainly by activating downstream signals such as Janus kinase signal transduction/transcriptional activator (JAK/STAT) and mitogen activated protein kinases (MAPK). Initially, IL-27 was considered to be an inflammatory factor with pro-inflammatory and anti-inflammatory effects. However, with the development of research in recent years, it has been confirmed that IL-27 plays a role in the body’s immunity and participates in osteogenesis and osteoclast differentiation, phagocytosis and improve insulin sensitivity. IL-27 mainly exerts immunosuppressive effects on CD4+ T cells, CD8+ T cells, and B cells. In CD4+ T cells, IL-27 has dual roles due to different stages of cell differentiation, local microenvironment changes, or activated downstream signals, e.g., IL-27 promotes Th1 cell differentiation in CD4+ T cells and can inhibit differentiation in a highly polarized state of Th1 cells. In CD8+ T cells, IL-27 promotes their proliferative activation and thus plays a role in the immune response and the efficacy of subunit vaccination. In B cells, due to different activation patterns and differentiation stages, IL-27 also has a dual role. It enhances the phosphorylation levels of STAT1 and STAT3 in naive B cells and promotes B cell proliferation; but in memory B cells, its effect is weakened and cannot promote B cell proliferation. In osteoblasts (OB) and osteoclasts (OC), IL-27 promotes OB differentiation mainly through Smad2/3 and STAT1, and inhibits OB differentiation through M-CSF/sRANKL in bone marrow-derived macrophage-like cells OC generation, thereby participating in the regulation of bone growth and remodeling. In addition, the latest research has confirmed that IL-27 can directly target adipocytes, promote adipocyte (AD) thermogenesis, and improve insulin sensitivity through the p38MAPK-PGC-1α signaling pathway, which also suggests that IL-27 plays an important role in metabolic great potential in disease. This article reviews the structure and biological function of IL-27, aiming to provide a reference for the research and application of IL-27.