The　short-chain　fatty　acids　(SCFAs)　production　was　facilitated　in　Na+　assistant　anaerobic　fermentation　of　waste　activated　sludge,　whereas　the　relevant　micro-ecosystem　characteristics　were　unclear.　This　work　explored　the　microbial　community　and　hydrolases　shifts　under　Na+　stress.　It　demonstrated　that　the　selective　Na+　stress　increased　protease　activities　to　126–160%　while　inhibiting　α-glucosidase　activities　to　83.1–91.3%.　Correspondingly,　the　biodegradation　rates　of　protein　and　glucose　were　improved　from　25.6%　and　45.8%　to　39.0%　and　55.2%,　respectively.　Furthermore,　the　microbial　species　richness　and　biodiversity　gradually　decreased,　attributing　to　Na+-induced　cell　lysis.　Selective　Na+　stress　was　the　prime　inducement　for　microbial　community　evolution　and　promoted　“SCFAs-producing”　bacterial　community　formation　by　“bacteria-screening”　roles.　Overall,　the　hydrolytic　bacteria　and　acidogens　became　dominant　bacteria,　which　were　resistive　to　Na+　stress,　while　the　SCFAs　consumers　were　restrained　as　they　are　salinity-sensitive.　Such　microbial　community　shift　modified　functional　metabolisms　for　enhancing　anaerobic　fermentation　performance.　The　pathways　for　metabolism　and　genetic　information　processing　were　improved,　positively　relating　to　the　facilitated　catabolism　of　fermentation　substrates.　Consequently,　the　mechanism　of　maximized　SCFAs　accumulation　by　selective　Na+　stress　was　proposed,　i.e.　microbial　modification　of　fermentation　ecosystem　for　facilitating　sludge　hydrolysis　and　acidification　with　impeded　methanogenesis.　©　2022　Elsevier　B.V.