Abstract: Based on the Liege intra-nuclear cascade (INCL) model, we study the impact of medium modifications in nucleon-nucleon elastic scattering on spallation reactions. In this study, we adopt two types of medium modification forms for the nucleon-nucleon scattering cross sections: phenomenological and microscopic. We find that for the reaction of proton with ^232\textTh, medium modifications can significantly reduce the production cross section of fission products at relatively lower energies, while the impact on fission products at higher energies is minor, but greatly affects the production cross section of multi-nucleon emission residual products. The effect of medium modifications on neutron differential cross sections shows similar results at different incident energies. We analyze this phenomenon and provide a preliminary explanation for its possible causes. Finally, we calculate the mass distribution of final state products for proton and ^197\textAu reaction at an incident energy of 400 MeV and compare with experimental results. The results corresponding to the medium modification factors for the nucleon-nucleon scattering cross section calculated using Brueckner-Hartree-Fock (BHF) theory align better with the experimental data.