Photonic　synapse　transistors　have　attracted　growing　attention　due　to　their　salient　advantages　in　information　transmission/processing　and　good　potentials　for　analog　neural　computing.　Currently,　most　synapse　transistors　are　rigid　devices　with　the　limitations　in　flexible　and　highly　photosensitive　semiconductor　channel　layers.　To　achieve　high-performance　flexible　photonic　synapse　transistors,　it　is　essential　to　develop　stimuli-responsive　organic　semiconductor　thin-films　to　broaden　the　photo-response　range　of　devices　and　accelerate　separation　of　photoinduced　carriers　for　improving　synaptic　performances.　Here,　poly(3-hexylthiophene)　(P3HT)　and　poly(indacenodithiophene-co-benzothiadiazole)　(PIDT-BT)　materials　with　complementary　optical　absorption　have　been　used　to　fabricate　a　bulk　composite　film　of　organic　semiconductor　heterostructures　by　a　solution-processing　method.　The　surface　structures　and　optoelectronic　properties　of　the　PIDT-BT:P3HT　film　and　their　single　component　films　are　studied.　By　using　the　PIDT-BT:P3HT　heterostructure　film　as　a　novel　photoactive　channel　layer,　flexible　low-voltage　photonic　synapse　transistors　have　been　designed　and　fabricated　with　the　combination　of　polyelectrolyte-based　dielectrics.　The　effects　of　different　optical　stimulation　conditions　on　the　performance　of　synapse　transistors　are　investigated　with　insightful　discussion　on　the　semiconductor　mechanisms.　The　flexible　synapse　transistors　based　on　the　PIDT-BT:P3HT　film　exhibit　higher　excitatory　postsynaptic　currents　than　devices　with　the　single　component　semiconductor　layers　at　the　same　optical　stimulation.　The　synaptic　response　of　the　PIDT-BT:P3HT-based　device　is　also　evaluated　under　different　stimulus　variables　including　the　optical　spike　wavelength,　duration　and　frequency.　As　results,　good　synaptic　characteristics　are　observed　including　the　high　excitatory　postsynaptic　current,　paired-pulse　facilitation,　and　frequency-dependent　properties　with　the　tunable　synaptic　plasticity.　It　is　found　that　the　response　in　excitatory　postsynaptic　current　of　the　synaptic　device　stimulated　by　two　beams　(520　nm　green　laser　and　685　nm　red　laser)　together　is　greater　than　the　sum　of　the　responses　stimulated　by　each　beam　alone.　These　results　are　useful　for　the　design　of　high-performance　photoresponsive　semiconductor　films　and　photonic　synapse　transistors,　which　are　conducive　to　the　development　of　low-power　flexible　optoelectronic　devices,　artificial　synapses　and　beyond.