Abstract: In order to study intensity distribution of a broadband laser with flattened-Gaussian transverse modes passing through a hard-edged aperture, the propagation expression of each frequency component of the broadband laser was deduced on the basis of the diffraction integral, and then the propagation expression in spatial domain was obtained by means of Fourier transform. Effect of the aperture and the bandwidth on the intensity distribution was analyzed. The results show that the intensity distribution keeps somewhat flattened profile and the profile become narrower with increasing bandwidth when the truncation parameter is large in the near field. When the truncation parameter is small, the intensity distribution is Gaussian profile and the bandwidth has little effect on the profile. In the far field, however, the profile of the intensity also presents narrowing effect with increasing bandwidth when the truncation parameter is small and the narrowing effect tends to decrease with increasing truncation parameter. The results have significant value for applications of broadband laser with flattened-Gaussian transverse mode.