Mid-infrared continuous-wave optical parametric oscillator (CW-OPO) is the focus in the field of nonlinear frequency conversion. It has important applications in optical communication, spectroscopy, remote sensing and infrared countermeasure. We analyzed the main features of the nonlinear crystal which is used to produce mid-infrared laser, and overviewed the experimental and theoretical researches of mid-infrared CW-OPO. We also proposed some of its future research directions. Currently, mid-infrared CW-OPO can achieve the output power from a hundred millwatts to ten watts in 3～ 5 μm with high stability, wide tunability. With the development of laser technology and periodically poled crystals with further improved performance, the next step is expected to achieve higher-power mid-infrared continuous-wave laser output in a larger tuning range.