Abstract: 3 μm~5 μm is one of the transmission windows of the atmosphere, including many atoms or molecules' characteristic absorption peaks. 3 μm~5 μm laser souses have wide applications, such as medical diagnosis, environmental monitoring, spectral analysis, military countermeasure, and so on. In these application fields, light sources are often required to have narrow linewidth and wavelength tuning capability. Narrow-linewidth laser is an ideal light source for these applications due to its small spectrum range and concentrative energy. The technology of narrow-linewidth of 3 μm~5 μm laser were summarized, including the linewidth compression method of the Fe²⁺/Cr²⁺ ion-doped solid-state laser and fluoride fiber laser, and several methods of quantum cascade laser frequency stabilization were discussed. The compact and all-solid-state mid-infrared optical parametric oscillator was mainly introduced. And the related research work of our group on the narrow linewidth of an optical parametric oscillator was introduced. Finally, the research prospect of the narrow-linewidth mid-infrared laser has been prospected.