Organic　phototransistors　with　high　responsivity　and　sensitivity　to　light　irradiance　have　great　potential　applications　in　environmental　monitoring,　space　exploration,　security,　image　sensors　and　healthcare　systems.　In　this　manuscript,　a　novel　polymer　bulk　heterojunction　field　effect　phototransistor　with　ultrashort　channel　length　(tens　of　nanometers)　and　ultrahigh　sensitivity　to　visible　light　was　proposed.　Due　to　the　nanoscale　channel　and　bulk　heterojunction　structure,　a　high-performance　phototransistor　with　high　responsivity　of　750　A/W,　photosensitivity　of　1.0　×　106,　and　detectivity　as　high　as　4.54　×　1015　Jones　was　demonstrated　under　720　nm　light　illumination　with　0.1　mW/cm2　intensity,　which　was　even　better　that　those　lateral　organic　phototransistors.　Moreover,　organic　phototransistors　with　ultrashort　channel　length　were　investigated　for　the　first　time　on　a　flexible　substrate,　which　exhibited　outstanding　mechanical　flexibility　due　to　their　unique　designs.　Further　investigation　of　the　correlation　between　the　morphology　of　bulk　heterojunction　blends　and　the　device　photoresponse　performance　indicated　that　the　photoelectric　properties　of　the　devices　could　be　effectively　enhanced　by　controlling　the　morphology　of　semiconducting　layers.　More　importantly,　this　work　provided　first　clear　experimental　evidence　that　the　surface　area　of　semiconducting　crystals　would　significantly　impact　dissociation　of　photoinduced　excitons　and　transfer　and　recombination　of　photogenerated　carriers,　which　is　crucial　for　photoelectric　performance　of　phototransistors.　This　work　provided　critical　guidelines　for　the　development　of　high　performance　flexible　organic　phototransistors,　which　opened　up　the　doors　of　opportunity　for　organic　phototransistors　applications　in　a　wide　range　of　organic　electronics.　©　2018　American　Chemical　Society.