IL-15 is a key molecule in immune regulation and is secreted by myeloid cells. IL-15 plays an important role in the homeostasis and function of T cells, natural killer (NK) cells, and memory CD8+ T cells. Due to its characteristics of wide expression and strict secretion, IL-15 holds significant therapeutic potential in various immune-related diseases. After IL-15 specifically binds to IL-15 receptor, it activates a variety of signaling pathways such as JAK/STAT, Ras/Raf/MAPK and PI3K/AKT, inducing cell proliferation, differentiation and apoptosis, thus exerting biological effects such as anti-tumor and anti-infection effects. This article reviews the role and associated mechanisms of IL-15 in diseases such as tumors, autoimmune diseases, cardiovascular diseases, and neuropsychiatric diseases, and summarizes the small molecule agonists and antagonists with IL-15 as potential therapeutic targets, aiming to provide a scientific basis for further investigation of the pathogenesis and drug research of IL-15 in immune system diseases and neuropsychiatric diseases.