The　industrial　manufacture　of　ammonia　(NH3)　using　Fe-based　catalyst　works　under　rigorous　conditions.　For　the　goal　of　carbon-neutrality,　it　is　highly　desired　to　develop　advanced　catalyst　for　NH3　synthesis　at　mild　conditions　to　reduce　energy　consumption　and　CO2　emissions.　However,　the　main　challenge　of　NH3　synthesis　at　mild　conditions　lies　in　the　dissociation　of　steady　N[tbnd]N　triple　bond.　In　this　work,　we　report　the　design　of　subnanometer　Ru　clusters　(0.8　nm)　anchored　on　the　hollow　N-doped　carbon　spheres　catalyst　(Ru-SNCs),　which　effectively　promotes　the　NH3　synthesis　at　mild　conditions　via　an　associative　route.　The　NH3　synthesis　rate　over　Ru-SNCs　(0.49%　(mass)　Ru)　reaches　up　to　11.7　mmol　NH3·(g　cat)−1·h−1　at　400　°C　and　3　MPa,　which　is　superior　to　that　of　8.3　mmol　NH3·(g　cat)−1·h−1　over　Ru　nanoparticle　catalyst　(1.20%　(mass)　Ru).　Various　characterizations　show　that　the　N2H4　species　are　the　main　intermediates　for　NH3　synthesis　on　Ru-SNCs　catalyst.　It　demonstrates　that　Ru-SNCs　catalyst　can　follow　an　associative　route　for　N2　activation,　which　circumvents　the　direct　dissociation　of　N2　and　results　in　highly　efficient　NH3　synthesis　at　mild　conditions.　©　2022　The　Chemical　Industry　and　Engineering　Society　of　China,　and　Chemical　Industry　Press　Co.,　Ltd.