The production cross sections for primary and residual fragments produced in two reaction systems $^{136}$Xe + $^{198}$Pt and $^{238}$U + $^{198}$Pt at incident energy 8 MeV/nucleon are calculated by improved quantum molecular dynamics (ImQMD) model incorporated with the statistical evaporation model (HIVAP code). It is found that the cross sections for n-rich $N=126$ residual fragments are several times larger in $^{238}$U$+{}^{198}$Pt than in $^{136}$Xe$+{}^{198}$Pt due to the larger cross sections and lower excitation energies of primary fragments in $^{238}$U induced reactions. In both systems, $N=126$ semi-magic residues are produced via neutron emission in the decay of primary fragments, which survive in the semi-central collisions and have lower excitation energies.