为了使谐波转换系统保持高效稳定的3次谐波转换，采用数值模拟的方法，研究了基频光脉冲波形对3倍频转换效率的影响，并对高斯脉冲和平顶脉冲的不同谐波转换特点作了详细分析。当基频光为高斯脉冲时，在3GW／cm2的输入条件下最佳的2倍频转换效率为56．8%，低于理论预计的66．7%。进一步计算了不同功率密度下，最佳2倍频转换效率与超高斯脉冲阶数的关系。当考虑空间走离效应，基频光时间和空间均为平顶分布时，最佳的2倍频转换效率为62%；若基频光时间为平顶分布、空间分布为高斯分布时，最佳2倍频转换效率为51%，进一步地偏离66．7%。结果表明，当基频光脉冲形状偏离理想的平顶分布时，适当地降低2倍频转换效率，可提高3倍频转换效率。

In order to achieve stable and high conversion efficiency,the influence of fundamental pulse shape on thirdharmonic generation was analyzed and different physical insights of third-harmonic conversion with flat-top pulse and Gaussian pulse were studied respectively. In the case of Gaussian fundamental pulse and 3GW/cm² input intensity,the calculated optimal second-harmonic conversion efficiency was 56.8%,lower than 66.7% predicated in theory. Furthermore,the dependence of optimal second-harmonic conversion efficiency on fundamental pulse shape was calculated under different input intensity. The spatial walk-off effect will influence the optimal second-harmonic conversion efficiency,lowering it to 62% in the case of fundamental pulse with flat-top spatial and temporal distribution. The optimal second-harmonic conversion efficiency will decrease to 51% when the spatial distribution of fundamental pulse is Gaussian. It is discovered that when the fundamental pulse is not ideal flat-top pulse shape,the third-harmonic conversion efficiency can be improved by lowering the second-harmonic conversion efficiency properly.