We present a comprehensive understanding of sing Ru atom supported on defective graphene for water splitting using density functional theory calculations and microkinetic analysis. The structural and electronic properties of Ru atom supported single vacancy graphene (SVG), double vacancy graphene (DVG) and Stone-Wales graphene (SWG) are systematically investigated. We find that the Ru atom can be trapped effectively by the defects on each defective graphene surfaces. The binding strength of the single Ru atom onto defective graphene surfaces follows the order: Ru@SVG > Ru@DVG > Ru@SWG. After binding, the d-band centers in Ru@SVG, Ru@DVG and Ru@SWG are about -1.67, -1.36, -1.02 eV, respectively. We find that the reaction barrier of H2O splitting decreases with increase of Ru d-band center. The reaction activity of H2O splitting are as follows: Ru@SVG < Ru@DVG < Ru@SWG. The splitting of water molecule on Ru@SWG surface only need a small activation energy of 0.43 eV. In addition, in the temperature range of 300-600 K, the Ru@SWG presents better reaction activity for H2O splitting. The reaction rate and turnover frequency are orders of magnitude larger than that on Ru@SVG and Ru@DVG. Overall, our study provides insights on the significant role of a single metal atom on defective graphene. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.