The smoothing scheme combining a diffraction-weakened lens array with the induced spatial incoherence method is proposed and demonstrated to be an efficient smoothing scheme for broadband laser systems. In our simulation, the RMS illumination nonuniformity of the target spot is reduced to 2% after sufficient smoothing time. The temporal characteristics and spatial power spectral density of the scheme are theoretically analyzed. When the incident light has intensity fluctuations, the uniformity of the target spot is stable, which means a robust smoothing scheme, and which predicts practical applications to the smoothing of broadband laser systems.