Abstract: A mathematical model for optical parametric oscillator (OPO) by using paraxial approximation was proposed. It could simulate the wave mixing process of both nanosencond waves and contiunes waves. This model including the dispersion of different wavelengths of the three mixing waves used the splitFourier method and the spatial and temporal relations between mixing waves to describe the process in OPO. The model could give output waveform, power and threshold in high efficiency OPO under the conditions of different pump pulse energies, pulse widths, transmissivities and reflectivities of cavity mirrors. In the experiment, a periodically poled lithium niobate crystal (PPLN) was used as the nonlinear medium. When the single pulse energy of input pump beam at 1.06 m was 0.4 mJ, we could get the energy of idle wave at 3.8 m more than 0.07 mJ, which was qualitatively fitted by the numerical result of 0.08 mJ.