In a multi-optical parametric oscillator by pulse pumping, energy conversion process for 1.57 μm and 3.84 μm parametric light can be expressed by time-dependent wave equations. The split-step integration method is used to solve the equations. By analyzing the simulation results of the output waveform for the multi-optical parametric amplifier, it is confirmed that back conversion and mode competition are the important factors affecting the multi-optical parametric oscillation. The 1.57 μm and 3.84 μm parametric light in an external cavity multi-optical parametric oscillator are simulated under different output mirror transmittances, crystal working lengths and cavity lengths. The conversion efficiency of 1.57 μm and 3.84 μm increase with the output mirror transmittance increasing, which means that the conversion efficiency can be adjusted by changing the parametric light transmittance of the output mirror. There exist an optimal crystal working length and a cavity length in the external cavity multi-optical parametric oscillator. The experiment on external cavity multi-optical parametric oscillator is carried out. The conversion efficiency of 1.57 μm and 3.84 μm parametric light are consistent with the theoretical values. The energy conversion process in the multi-optical parametric oscillator can be simulated by this method, which could be used for optimizing the multi-optical parametric oscillator and increasing the parametric conversion efficiency.