The electrocatalytic performance of nitrogen reduction reaction (NRR) is seriously hindered by the lack of cost-effective electrocatalysts with high-efficiency and high-selectivity. In this work, the NRR catalytic activity of single carbon (C) atom embedded into two-dimensional (2D) transition metal carbides (M2CO2, M = Ti, Zr, Hf, Nb, Ta, Mo, and W) with oxygen vacancy was systematically evaluated by means of comprehensive density functional theory (DFT) computations. Our results revealed that the embedded single C atom possesses good durability due to its strong interaction with metal atoms around vacancy in these MXenes. Interestingly, through high-throughput screening, the single C atoms anchored on Nb2CO2, Mo2CO2, and W2CO2 nanosheets are identified as promise NRR catalysts with high-activity due to their low limiting potentials (-0.14 to -0.38 V) via a distal mechanism and outstanding selectivity again the competing hydrogen evolution reaction. Remarkably, the intrinsic activity of the C single atom supported by these MXenes mainly originates from the activation degree of the adsorbed N-2 molecule, which is greatly dependent on the electron filling degree of p(z) orbital in C atom. Thus, by carefully choosing suitable substrates, the single C catalyst can be utilized as ideal NRR catalysts for NH3 synthesis. (C) 2020 Elsevier Inc. All rights reserved.