A 3–5 μm mid-infrared band is a good window for atmospheric transmission. It has the advantages of high contrast and strong penetration under high humidity conditions. Therefore, it has important applications in the fields of laser medicine, laser radar, environmental monitoring, remote sensing, molecular spectroscopy, industrial processing, space communication and photoelectric confrontation. In this paper, the application background of mid-infrared laser is summarized. The ways to realize mid-infrared laser output are described by optical parametric oscillation, mid-infrared solid-state laser doped with different active ions and fiber laser doped with different rare earth ions. The advantages and disadvantages of various mid-infrared lasers are briefly described. The technical approaches, schemes and research status of mid-infrared lasers are introduced.