Peracetic　acid　(PAA)　combined　with　free　ammonia　(FA)　pretreatment　can　be　utilized　to　promote　anaerobic　fermentation　(AF)　of　waste　activated　sludge　(WAS)　to　produce　short-chain　fatty　acids　(SCFAs),　and　the　resulting　SCFAs　are　desirable　carbon　sources　(C-sources)　for　polyhydroxyalkanoate　(PHA)　biosynthesis.　This　work　aimed　to　determine　the　optimum　conditions　for　PAA　+　FA　pretreatment　of　sludge　AF　and　the　feasibility　of　using　anaerobic　fermentation　liquor　(AFL)　for　PHA　production.　To　reveal　the　mechanisms　of　integrated　pretreatment,　the　impacts　of　PAA　+　FA　pretreatment　on　different　stages　of　sludge　AF　and　changes　in　the　microbial　community　structure　were　explored.　The　experimental　results　showed　that　the　maximum　SCFA　yield　reached　491.35　+/-　6.02　mg　COD/g　VSS　on　day　5　after　pretreatment　with　0.1　g　PAA/g　VSS　+70　mg　FA/L,　which　was　significantly　greater　than　that　resulting　from　PAA　or　FA　pretreatment　alone.　The　mechanism　analysis　showed　that　PAA　+　FA　pretreatment　promoted　sludge　solubilization　but　strongly　inhibited　methanogenesis.　According　to　the　analysis　of　the　microbial　community,　PAA　+　FA　pretreatment　changed　the　microbial　community　structure　and　promoted　the　enrichment　of　bacteria　related　to　hydrolysis　and　acidification,　and　Proteiniclasticum,　Macellibacteroides　and　Petrimonas　became　the　dominant　hydrolytic　and　acidifying　bacteria.　Finally,　after　alkali　treatment,　the　AFL　was　utilized　for　batch-mode　PHA　production,　and　a　maximum　PHA　yield　of　55.05　wt%　was　achieved　after　five　operation　periods.