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The performance of lithium-ion batteries (LIB) with liquid electrolytes is strongly dependent on a stable solid electrolyte interphase (SEI) film formation on the anode surface. According to recent experiment studies, the use of highly concentrated (HC) electrolyte can be quite useful to improve the battery performance, enhancing the SEI film formation. However, its molecular mechanism remains still unknown. To investigate such film formation mechanism, we performed the atomistic reaction simulations in acetonitrile (AN)-based electrolyte solutions using the Red Moon method (a hybrid Monte Carlo (MC)/molecular dynamics (MD) reaction method). The present simulations were able to successfully reproduce the experimental observations where the reaction products produced by the reduction of salts mainly form the SEI film in the HC electrolyte. Further, it was revealed that such stable SEI film can be formed in a stepwise fashion: (i) the diffusive transfer of reduction products, (ii) the Li salt-based passivation film formation, and (iii) the formation of the solvent-based film layer. This new microscopic insight should provide an important guiding principle in designing the most effective electrolytes to develop high-performance LIB with the HC electrolyte.