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 O 2 = 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, CH 3 CHO, CH 3 CH 2 OCO) on Cu and Cu 2 O 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.