Synthesis of diethyl carbonate (DEC) in the liquid phase by oxidative carbonylation of ethanol over activated carbon (AC) supported chloride-free Cu-based catalysts offers a prospective "green chemistry" strategy compared to the traditional preparation processes. The catalysts of Cu/AC were synthesized via a one-pot carbothermal method, and the Cu(+1) and Cu(0) valence distribution of Cu/AC catalyst was adjusted by simple carbon heat treatment. The physicochemical properties of the catalysts of Cu/AC were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and the Brunauer-Emmett-Teller model. The 10%-Cu/AC-873 catalyst (calcined at 873 K) exhibits the best catalytic activity for the synthesis of DEC under optimized conditions: T = 393 K, P = 4.0 MPa, P-CO/P-O2 = 9:1, and reaction time 2 h. The conversion of ethanol and space-time yield of DEC were 7.3% and 594.4 mg.g(-1).h(-1) , respectively. The 10%-Cu/AC-873 catalyst was reused four times without an obvious decrease in the activity. The adsorption energies of reactive species (CO, CH3CHO, CH3CH2OCO) on Cu and Cu2O were calculated by density functional theory; the results indicate that the synergic function of Cu+ and Cu-0 enhanced the catalytic performance of Cu-based catalyst.