Abstract: The propagation properties of broadband laser with flattened-Gaussian transverse modes are studied and the influences of the laser bandwidth on intensity distributions are analyzed. The propagation expression of each frequency component of the broadband laser with flattened-Gaussian transverse modes is deduced and then the propagation expression in spatial domain is obtained by Fourier transform. Numerical examples are illustrated to show intensity distributions with different distance and bandwidth. Variations of spatial modes and spatiotemporal effect are investigated and discussed in detail. The results show that the intensity distributions tend to Gaussian mode with increasing of distance. Also, the laser presents stronger spatiotemporal effect when the bandwidth increases and the transverse distributions change evidently. The characteristic of the changes is that the intensity distributions of the wave front are narrowing and those of the wave rear are spreading with decreasing bandwidth.