Ammonia　is　an　important　feedstock　for　producing　fertilizer　and　chemicals　as　well　as　is　also　a　potential　energy　carrier.　Generally,　MgO　is　considered　as　an　electronic　promoter　for　Ru　catalysts　in　ammonia　synthesis.　In　this　research,　it　was　found　for　the　first　time　that　surface　atoms　of　Ru　particles　are　stabilized　by　MgO　modification　at　high　temperature　and　reaction　atmosphere,　which　contained　more　highly　active　B5-sites　for　ammonia　synthesis.　Ex　situ　HRTEM　images　show　similar　Ru　nanoparticles　containing　steps　with　or　without　promoters.　Under　reaction　temperatures　and　gas　environment,　in　situ　environmental　transmission　electron　microscopy　(ETEM)　observe　that　Ru　particles　without　MgO　promoter　suffer　from　dynamic　change　and　lost　the　B5-sites　on　Ru　steps,　which　results　in　the　poor　ammonia　yield　obtained　by　both　the　continuing-flow　reaction　system.　While　MgO　modification　can　stabilize　the　Ru　surface　structure　and　keep　B5-sites　there,　which　contributes　to　the　much　higher　ammonia　yield.　Density　functional　theory　calculations　show　that　B5-site　is　a　catalytic　active　center　for　ammonia　synthesis　and　the　N-2　dissociation　barrier　on　B5-site　containing　step　is　lower　than　B5-site-free　step　on　Ru　nanoparticles.　The　finding　provides　a　new　explanation　for　the　high　activity　of　MgO　modified　Ru　catalysts.　Moreover,　adding　CsOx　with　MgO　will　contribute　electron　donation　to　stabilized　Ru　surface　and　accelerate　the　ammonia　synthesis　with　five-fold　higher　than　MgO-Ru/MS　at　500　degrees　C.　A　better　fundamental　understanding　of　the　dynamics　of　Ru　catalysts　and　its　reaction　mechanism　in　this　research　is　vital　for　optimizing　the　design　of　catalysts　for　ammonia　synthesis.