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 <math><mi>N</mi><mo>=</mo><mn>126</mn></math> residual fragments are several times larger in $^{238}$U<math><mo>+</mo><msup><mrow></mrow><mrow><mn>198</mn></mrow></msup></math>Pt than in $^{136}$Xe<math><mo>+</mo><msup><mrow></mrow><mrow><mn>198</mn></mrow></msup></math>Pt due to the larger cross sections and lower excitation energies of primary fragments in $^{238}$U induced reactions. In both systems, <math><mi>N</mi><mo>=</mo><mn>126</mn></math> 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.