Differential absorption lidar (DIAL) is an important equipment for the detection of noxious gases. It has a high demand for the wavelength and linewidth of the laser sources. In recent years, optical parametric oscillation and amplification (OPO/OPA) technology has made breakthroughs in medium and long-wave infrared laser, showing great potential in obtaining high-quality long-wave laser. The characteristics of different nonlinear crystals are collected. The performance of long-wave laser obtained by some crystal optical parametric oscillators is generalized, including output power, pulse energy, tuning range and output linewidth. Combined with the theoretical gain linewidth calculation and recent experimental research, the main problems in realizing narrow linewidth are analyzed and summarized, furthermore, the future technical route furthermore,the development direction are prospected.