2 μm, 3-5 μm and 8-12 μm infrared lasers are located in the atmospheric transmission window, which have broad applications in laser medicine, laser imaging, environmental monitoring, lidar, chemical remote sensing and infrared countermeasure. Based on the optical nonlinear frequency conversion technology and nonlinear optical crystals, it has obvious advantages in achieving middle-long-wave infrared solid lasers, such as compact and simple structure, wide tunable wavelength range and high output power. Using ZnGeP₂ crystal with 2 μm Ho single-doped solid laser as pump especially has an outstanding performance in middle-long-wave infrared field. In the aspect of average output power, it has reached the level of 102 W@3-5 μm, 12.6 W@8.2 μm and 3.5 W@9.8 μm. Moreover, they all have a beam quality factor M² less than 3 and the corresponding optical-to-optical conversion efficiency of 3-5 μm is about 60%. This paper reviewed the research progress of 2 μm Ho single-doped solid laser and application of ZnGeP₂ on middle-long-wave infrared in detail.