A stable and reliable optical parametric oscillator (OPO) with the best coherence and tunable spectral range is an ideal laser source. It is widely used in spectrum analysis, laser interference measurement, remote sensing, laser differential absorption radar, etc. The structure of resonant cavity, focusing parameters, pump threshold ratio, thermal effect, group velocity dispersion and other nonlinear effects affect the stability of the OPO system. Various factors which influence the stability of optical parametric transformation are analyzed, the most effective way to eliminate the noise of the OPO system is pointed out, which is the realization of single frequency operation.