Single　atom　catalyst　(SAC)　is　one　of　the　most　efficient　and　versatile　catalysts　with　well-defined　active　sites.　However,　its　facile　and　large-scale　preparation,　the　prerequisite　of　industrial　applications,　has　been　very　challenging.　This　dilemma　originates　from　the　Gibbs-Thomson　effect,　which　renders　it　rather　difficult　to　achieve　high　single　atom　loading　(＜　3　mol%).　Further,　most　synthesizing　procedures　are　quite　complex,　resulting　in　significant　mass　loss　and　thus　low　yields.　Herein,　a　novel　metal　coordination　route　is　developed　to　address　these　issues　simultaneously,　which　is　realized　owing　to　the　rapid　complexation　between　ligands　(e.g.,　biuret)　and　metal　ions　in　aqueous　solutions　and　subsequent　in　situ　polymerization　of　the　formed　complexes　to　yield　SACs.　The　whole　preparation　process　involves　only　one　heating　step　operated　in　air　without　any　special　protecting　atmospheres,　showing　general　applicability　for　diverse　transition　metals.　Take　Cu　SAC　for　an　example,　a　record　yield　of　up　to　3.565　kg　in　one　pot　and　an　ultrahigh　metal　loading　16.03　mol%　on　carbon　nitride　(Cu/CN)　are　approached.　The　as-prepared　SACs　are　demonstrated　to　possess　high　activity,　outstanding　selectivity,　and　robust　cyclicity　for　CO2　photoreduction　to　HCOOH.　This　research　explores　a　robust　route　toward　cost-effective,　massive　production　of　SACs　for　potential　industrial　applications.