Graphene　has　unique　advantages　in　ultrabroadband　detection.　However,　nowadays　graphene-based　photodetectors　cannot　meet　the　requirements　for　practical　applications　due　to　their　poor　performance.　Here,　we　report　a　graphene-silicon-graphene　Schottky　junction　photodetector　assisted　by　field　effect.　Two　separate　graphene　sheets　are　located　on　both　sides　of　the　n-doped　silicon　to　form　two　opposite　lateral　series　heterojunctions　with　silicon,　and　a　transparent　top　gate　is　designed　to　modulate　the　Schottky　barrier.　Low　doping　concentration　of　silicon　and　negative　gate　bias　can　significantly　raise　the　barrier　height.　Under　the　combined　action　of　these　two　measures,　the　barrier　height　increases　from　0.39　eV　to　0.77　eV.　Accordingly,　the　performance　of　the　photodetector　has　been　greatly　improved.　The　photoresponsivity　of　the　optimized　device　is　2.6　A/W　at　792　nm,　1.8　A/W　at　1064　nm,　and　0.42　A/W　at　1550　nm,　and　the　on/off　photo-switching　ratio　reaches　10(4).　Our　work　provides　a　feasible　solution　for　the　development　of　graphene-based　optoelectronic　devices.