A pulsed optical parametric oscillator model based on MgO-doped periodically poled lithium niobate crystal is established. The influence of seed intensity on conversion efficiency of the signal in the optical parametric oscillator is numerically investigated. The simulation results show that increasing the seed intensity can restrain multi-longitudinal-mode operation, and promote the conversion efficiency of the seed longitudinal mode remarkably. The conversion efficiency can not be promoted because of gain saturation if the seed intensity reaches the order of μW/mm2. The upper limit of gain saturation is affected by back-conversion, which leads to insufficient utilization of the pump energy. A prospect is given that back-conversion can be avoided in an optical parametric amplifier with a simpler structure by optimizing the crystal length. The pump energy thus can be utilized more sufficiently and the conversion efficiency of the single longitudinal mode is expected to be improved further.