Ammonia (NH3) is one of the most important building blocks of the chemical industry and a promis-ing sustainable energy carrier.Conventional production of NH3 via the Haber-Bosch process requires high temperature and high pressure,which is energy demanding and suffers safety issues.Photocatalytic nitro-gen reduction reaction (NRR) is a green and sustainable route for NH3 production,and has been expected to be an alternative for NH3 production under mild conditions.However,solar-driven N2 activated has ap-peared as the bottleneck for photocatalytic NRR.In this work,we propose that single Ru atom supported by BeO monolayer is a promising photocatalytic single atom catalyst (SAC) for efficient N2 activation with visible illumination.The high efficiency originates from the enhanced absorption in the visible range,as well as the back-donation mechanism when N2 were adsorbed on the SAC.Our results show that N2 can be efficiently activated by the Ru/BeO SAC and be reduced to NH3 with extremely low limiting potential of-0.41 V.The NRR process also exhibits dominate selectivity respect to hydrogen evolution.