Abstract: Mid-infrared all-solid-state optical parametric oscillator, due to wide tuning range, low threshold, high efficiency, high repetition rate and the advantages of miniaturization, have widespread optoelectronic countermeasures, laser radar, laser ranging, spectral measurements and the areas of environmental monitoring. In recent years there has been increasing interest in the mid-infrared OPO. This article focuses on research and progress of middle-infrared single resonant OPO（ISRO）, which is based on birefringent phase-matching crystals (such as KTA, KTP) and quasi-phase matching crystals (such as PPKTP, PPKTA, PPLN). We also discuses the development trend of ISRO.