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Flexible photodetectors show great applications in wearable electronics such as health monitoring and the Internet of Things. In this study, we have prepared ultralong ZnSe–PbSe heterojunction nanowires with a length of 20 μm in gram-scale by an in-situ cation exchange reaction. The produced PbSe nanoparticles on the ZnSe nanowire configuration are close to an ideal heterojunction system due to the small lattice mismatch of 7.4% in the (111) direction. The Ohmic contact is formed for nanowire photodetectors. The ZnSe–PbSe heterojunction nanowire photodetector exhibits a broadband photoresponse with the peak responsivity of 40 A W−1 while the ZnSe nanowire photodetector shows a narrowband photoresponse of 5 A W−1, as a result of its extended absorption across the entire UV-visible-NIR. Electrons are transported from PbSe to ZnSe while holes are trapped at PbSe, and electrons are extracted to an external circuit by an Al electrode, resulting in a high gain of the photoconductor. The broadband photoresponse, the large linear dynamic range of 66 dB, the fast response speed of 0.02 s and the protracted cycle of 5000 show the feasibility of the flexible heterojunction photodetector.