A numerical simulation model based on the cross-linked distribution of mitigated pits in the fused silica elements is proposed. The modulation effect of the mitigation area on the downstream light transport under the 351 nm laser irradiation is studied by the scalar diffraction theory and the fast Fourier transform algorithm. The results indicate that the modulation of the cross-linked mitigation area on the downstream light transport is mainly determined by the cross-linking degree of mitigated pits. With the increase of cross-linking degree, the local maximum of light modulation first increases rapidly and then decreases very fast, and the corresponding location first approaches and then is far away from the output surface of the mitigated element. With the increase of propagation distance, the light modulation first increases rapidly and then decreases very fast. The parameter optimization of the morphological structure of the mitigation area can avoid effectively the cascading damages among the downstream elements.