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The cooper pairs in the heavy-fermion superconductor CeCu2Si2 are formed of heavy fermions.Therefore,the heavy fermions are fundamental to the emergence of unconventional superconductivity and associated non-Fermi-liquid behavior in the normal state.The interplay between localization and itinerancy manifested on the electronic structure is key for understanding the heavyfermion behavior.Here,via the first-principle density functional theory (DFT) combined with single-site dynamical mean-field theory (DMFT),we investigate the temperature (T) evolution of the electronic structure ofCeCu2Si2 in the normal state,focusing on the role of the 4fstates in the low energy regime.Two characteristic temperature scales of this evolution,which accompanied the heavy-fermion formation,are established.The coherence onset temperature is around 130 K,whereas the heavy-fermion band formation temperature is between 40 and 80 K;both characteristic temperature scales are higher than the transport coherence temperature.Furthermore,the heavy-fermion formation is confirmed by calculating its effective mass variation with the temperature.Based on the calculated T-dependent evolution of the 4f orbital occupancy and electronic structure,an explanation on the behavior of the temperature evolution of the correlation strength of CeCu2Si2 is provided.Our results offer a comprehensive microscopic picture of the heavy-fermion formation in CeCu2Si2,which is essential for further understanding the emergent superconducting pairing mechanism.