Flexible　organic　synaptic　transistors　(FOSTs)　have　attracted　considerable　attention　owing　to　their　flexibility,　biocompatibility,　ease　of　processing,　and　reduced　complexity.　However,　FOSTs　rarely　maintain　the　mechanical　stability　of　their　synaptic　properties　while　meeting　the　device　deformation　requirements.　Here,　we　experimentally　found　that　bending　deformation　had　a　greater　influence　on　the　synaptic　performance　(i.e.,　the　excitatory　postsynaptic　current　(EPSC)　value)　of　FOSTs　than　on　the　on-state　current.　Moreover,　through　formula　derivation,　we　proved　that　the　density　of　bending-induced　defect　states　generated　near　the　channel　considerably　influences　the　synaptic　performance.　We　propose　a　general　approach　to　tune　the　stable　segment　of　the　device　using　an　encapsulation　layer.　The　EPSC　value　of　the　ordinary　FOSTs　without　a　regulated　stable　segment　decreased　by　nearly　1.5–2　orders　of　magnitude　after　bending.　In　contrast,　the　designed　flexible　synaptic　device　exhibited　relatively　stable　EPSC.　Moreover,　the　designed　FOST　exhibited　stable　paired-pulse　facilitation,　long-term　potentiation,　and　optical　synaptic　performance.　Furthermore,　neuromorphic　computational　simulations　based　on　our　device　before　and　after　500　bending　cycles　were　performed　using　a　handwritten　artificial　neural　network.　The　device　showed　stable　recognition　accuracy　after　50　learning　cycles　(91.55%　in　the　initial　state　and　90.43%　after　500　bending　cycles).　The　successful　application　of　a　stable　segment　in　flexible　synaptic　transistors　provides　a　convenient　and　simple　idea　for　fabricating　flexible　neuromorphic　electronics　with　mechanical　stability.[Figure　not　available:　see　fulltext.]　©　2023,　Science　China　Press.