This　study　focused　on　the　optimal　concentration　of　yeast　mixed　with　acetic　bacteria　to　promote　sludge　hydrolysis　and　acidification.　The　concentrations　of　orthophosphate,　ammonia　nitrogen　and　soluble　COD　released　from　sludge　were　investigated.　The　changes　of　protein　and　polysaccharide　in　extracellular　polymeric　substances　(EPS)　at　different　layers　were　also　studied.　At　yeast　and　acetic　acid　bacteria　dosage　of　10　and　20g/L,　the　highest　short　chain　fatty　acids　(SCFAs)　production　of　719mgCOD/gVSS　was　achieved　on　the　fifth　day　of　fermentation,　in　which　the　acetic　acid　was　328.78mgCOD/gVSS,　accounting　for　45.72%　of　the　total　SCFAs.　Adding　exogenous　bacteria　significantly　promoted　the　hydrolysis　of　excess　sludge　to　produce　SCFAs　with　acetic　acid　as　the　main　component.　The　addition　of　exogenous　bacteria　promoted　the　release　of　ammonia　nitrogen　and　orthophosphate　during　hydrolysis　acidification.　The　maximum　release　of　ammonia　nitrogen　and　orthophosphate　was　80.66mg/gVSS　and　22.38mg/gVSS　under　the　optimal　conditions,　respectively,　which　was　conducive　to　the　recovery　of　nitrogen　and　phosphorus　from　excess　sludge.　The　release　of　proteins　and　polysaccharides　from　the　inner　layer　to　the　outer　layer　of　EPS　was　improved,　providing　substrates　for　acid　production.　Exogenous　addition　of　yeast　and　acetic　acid　bacteria　was　an　effective　means　to　promote　hydrolysis　and　acidification　of　excess　sludge.　©　2020,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.