Abstract: In order to achieve stable and high conversion efficiency,the influence of fundamental pulse shape on third-harmonic 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.