A solid electrolyte interphase (SEI) plays an essential role in the functionality and service life of ion batteries, where the structure and formation mechanism are still in the midst. Here, we investigate the initial decomposition and reactions of ethylene carbonate (EC) on the surface of a graphite anode using first-principles calculations. EC initially decomposes via the homolytic ring opening with the product of radical anion CH2CH2OCO2−. Bonding with Li, it forms a co-plane structure of CH2CH2OCO2Li, with a binding energy of 1.35 eV. The adsorption energy is −0.91 eV and −0.24 eV on the graphite zigzag edge surface and basal surface, respectively. Two CH2CH2OCO2Li molecules react to form a two-head structure of lithium ethylene dicarbonate (CH2OCO2Li)2, namely LEDC, which further forms a network preferring zigzag edge surfaces. Our results suggest that the first and innermost layers of the solid electrolyte interphase are CH2CH2OCO2Li sticking and networking on the zigzag edges of the surfaces of graphite anodes.