Graphene　is　an　ideal　material　for　flexible　optoelectronic　devices　due　to　its　excellent　electrical　and　optical　properties.　However,　the　extremely　high　growth　temperature　of　graphene　has　greatly　limited　the　direct　fabrication　of　graphene-based　devices　on　flexible　substrates.　Here,　we　have　realized　in　situ　growth　of　graphene　on　a　flexible　polyimide　substrate.　Based　on　the　multi-temperature-zone　chemical　vapor　deposition　cooperated　with　bonding　a　Cu-foil　catalyst　onto　the　substrate,　the　growth　temperature　of　graphene　was　controlled　at　only　300　degrees　C,　enabling　the　structural　stability　of　polyimide　during　growth.　Thus,　large-area　high-quality　monolayer　graphene　film　was　successfully　in　situ　grown　on　polyimide.　Furthermore,　a　PbS-graphene　flexible　photodetector　was　fabricated　using　the　graphene.　The　responsivity　of　the　device　reached　10(5)　A/W　with　792　nm　laser　illumination.　The　in-situ　growth　ensures　good　contact　between　graphene　and　substrate;　therefore,　the　device　performance　can　remain　stable　after　multiple　bending.　Our　results　provide　a　highly　reliable　and　mass-producible　path　for　graphene-based　flexible　devices.