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The magic numbers were successfully explained by the nuclear shell model proposed by Gopper-Mayer and Jensen sixty years ago.The shell model predicts that the next doubly magic nucleus in the sequence will contain either 114,120,124 or 126 protons and 184 neutrons.However,the dynamical process of the formation of superheavy nucleus is not understood well enough.In this work,by using an improved isospin dependent molecular dynamics model in which the shell correction energy of the system is calculated by using deformed two-center shell model and the surface energy of the system is improved by introducing a switch function that combines the surface energies of projectile and target with the one of the compound nucleus,the effects of the shell correction energy on synthesis of superheavy nuclei and the fusion cross sections in asymmetric and nearly symmetric reaction systems leading to the same compound nuclei are studied.The entrance channel mass asymmetry dependence of compound nucleus formation is found by analyzing the shell correction energies,Coulomb barriers and fusion cross sections.The calculated results are in agreement with the experimental data and it is found that the compound nucleus formation is favorable for the systems with larger mass asymmetry.