2-5 μm mid-infrared laser has important applications in directional infrared countermeasures, anti-terrorism, biological medical treatment, environmental monitoring, optical communications, strong field physics and laser fusion et al. It can also be used as pump source for 5-20 μm mid- and far-infrared laser output. The study from Nd3+ and Yb3+ at 1 μm, Er3+ at 1.5 μm, Tm3+ and Ho3+ at 2 μm, Tm3+ and Cr2+ at 2-3 μm, Er3+, Ho3+, U4+and Dy3+ at 3 μm, Fe2+and Ho3+ at 4 μm, to Dy3+, Er3+and Pr3+ beyond 4 μm, continuously expands to the mid-wave infrared direction, which is the frontier of laser crystal development. In this paper, the research of mid-infrared laser crystals in recent years is reviewed, and its future development is commented briefly. The work of some transparent ceramics was involved.