A tunable single-frequency optical parametric oscillator (OPO) based on quasi-phase-matching is an ideal coherent light source with, high efficiency, good coherence, wide tunable spectral range and reliable and stable performance. It is widely used in laser interference measurement, laser differential absorption radar, spectral analysis, optoelectronic countermeasure, laser medicine etc. Due to the wide optical parametric gain and high power pump, most OPOs are often operated in multiple longitudinal modes, and a variety of methods are needed to achieve single-frequency operation. These methods include frequency selection method (such as injection-seed, grating and Fabry-Pérot etalon) and the operation characteristics of the OPO system itself (pump threshold ratio, thermal self-locking effect and thermal waveguide effect). This paper analyzes the realization of tunable single-frequency OPO.