By using Fourier optics, a model based on the far field about the Gauss pulse is set up，by which the effect of seams on the far field can be studied. Numerical simulation shows that pulse-width narrowing will appear in the case of single seam and the Fourier-translation-limited pulse-width of the input pulse will determine whether the pulse width in the far field becomes narrow or broad. In addition, the irradiance in the focus decreases and two sub-peaks appear beside the central peak, and their intensties increase with the widening of seams.