The　elastic　fiber　prestressing　(EFP)　technique　has　been　developed　to　balance　the　thermal　residual　stress　generated　during　curing　of　a　polymeric　composite,　where　continuous　fibers　were　prestretched　under　either　constant　stress　or　constant　strain　throughout　the　curing　process.　The　tension　was　only　removed　after　the　resin　was　fully　cured.　It　has　been　demonstrated　that　EFP　is　able　to　enhance　the　shear　properties　of　the　composite,　while　the　underlying　mechanics　is　still　unknown.　Here,　we　investigated　the　multiscale　shear　failure　mechanisms　induced　by　the　EFP　within　a　carbon　composite.　A　bespoke　biaxial　fiber　prestressing　rig　was　developed　to　apply　biaxial　tension　to　a　plain-weave　carbon　prepreg,　where　the　constant　strain-based　EFP　method　was　employed　to　produce　prestrained　composites　with　different　prestrain　levels.　Effects　of　EFP　on　macro-scale　shear　failure　were　subsequently　characterized　through　mechanical　tests　and　micro-morphological　analysis.　Both　the　micro-　and　meso-scale　representative　volume　element　(RVE)　finite　element　models　were　established　and　experimentally　verified.　These　were　then　employed　to　reveal　the　underlying　stress　evolution　mechanics　induced　by　EFP.　It　is　found　that　EFP　would　improve　the　shear　performance　of　a　composite　by　enhancing　the　fiber/matrix　interfacial　bonding　strength.　This　attributes　to　the　elastic　strain　recoveries　of　the　prestrained　fibers　locked　within　a　polymeric　composite,　which　generate　compressive　stresses　to　counterbalance　the　external　loading.　The　multiscale　shear　failure　mechanisms　were　then　proposed.　These　findings　are　expected　to　facilitate　structural　design　and　application　of　the　EFP　for　aerospace　composites.　Highlights:　Biaxial　tension　is　applied　to　produce　prestrained　woven　composite.　Prestrain　effects　on　microstructural　stress　evolution　mechanics　are　revealed.　Multiscale　shear　failure　mechanisms　are　proposed　for　prestrained　composites.　©　2024　Society　of　Plastics　Engineers.