Free　nitrous　acid　(FNA)　pretreatment　and　alkyl　polyglucoside　(APG)　addition　are　environmentally-friendly　methods　for　enhancing　waste　activated　sludge　(WAS)　anaerobic　fermentation　(AF)　performances.　The　short-chain　fatty　acids　(SCFAs)　produced　are　available　substrates　for　polyhydroxyalkanoates　(PHAs)　synthesis　to　reduce　the　C-source　cost.　This　study　aims　to　investigate　the　optimal　fermentation　conditions　of　FNA　+　APG　treatment　and　the　performance　of　PHAs　synthesis　by　the　SCFAs　from　AF　system.　Through　investigating　the　FNA　+　APG　combined　treatment　effects,　key　enzyme　activities,　and　microbial　community　variation,　the　AFL　from　optimal　fermentation　condition　was　utilized　as　a　C-source　for　PHAs　synthesis.　Experimental　results　indicated　that　0.653　mg/L　FNA　+　0.075　g/g　VSS　APG　treatment　could　synergistically　achieve　the　maximum　total　SCFAs　yield　of　324.94　mg　COD/g　VSS　at　day　5　of　AF,　which　was　much　more　than　those　of　sole　FNA　or　APG　treatment.　Mechanism　analysis　indicated　that　FNA　+　APG　treatment　could　promote　WAS　solubilization,　hydrolysis　and　acidogenesis,　while　severely　inhibiting　methanogenesis.　Microbial　community　analysis　results　demonstrated　that　SCFA　producers,　including　Enterococcus　and　Clostridium,　became　the　dominant　genera　in　the　FNA　+　APG　reactor.　Finally,　AF　liquor　after　ammonia　removal　was　applied　for　batch-mode　PHAs　synthesis,　and　PHA　yields　increased　to　a　maximum　of　44.44　%　of　biomass　(w/w)　after　five　operation　periods.