Peracetic　acid　(PAA),　known　for　its　environmentally　friendly　properties　as　a　oxidant　and　bactericide,　is　gaining　prominence　in　decontamination　and　disinfection　applications.　The　primary　product　of　PAA　oxidation　is　acetate　that　can　serve　as　an　electron　acceptor　(EA)　for　the　biosynthesis　of　medium-chain　fatty　acids　(MCFAs)　via　chain　elongation　(CE)　reactions.　Hence,　PAA-based　pretreatment　is　supposed　to　be　beneficial　for　MCFAs　production　from　anaerobic　sludge　fermentation,　as　it　could　enhance　organic　matter　availability,　suppress　competing　microorganisms　and　furnish　EA　by　providing　acetate.　However,　such　a　hypothesis　has　rarely　been　proved.　Here　we　reveal　that　PAA-based　pretreatment　leads　to　significant　exfoliation　of　extracellular　polymeric　substances　(EPS)　from　sludge　flocs　and　disruption　of　proteinic　secondary　structures,　through　inducing　highly　active　free　radicals　and　singlet　oxygen.　The　production　of　MCFAs　increases　substantially　to　11,265.6　mg　COD　L−1,　while　the　undesired　byproducts,　specifically　long-chain　alcohols　(LCAs),　decrease　to　723.5　mg　COD　L−1.　Microbial　activity　tests　further　demonstrate　that　PAA　pretreatment　stimulates　the　CE　process,　attributed　to　the　up-regulation　of　functional　genes　involved　in　fatty　acid　biosynthesis　pathway.　These　comprehensive　findings　provide　insights　into　the　effectiveness　and　mechanisms　behind　enhanced　MCFAs　production　through　PAA-based　technology,　advancing　our　understanding　of　sustainable　resource　recovery　from　sewage　sludge.　©　2023　The　Authors