A novel heterojunction ultraviolet (UV) photodetector of assembling Ag nanoparticles (NPs) onto ZnO nanowire (NW) arrays was fabricated via combination of chemical vapor deposition and thermal evaporation route. The fabricated composite Ag@ZnO NW arrays show blue-shift of UV peaks, suppression of the visible peaks, and obvious enhancements in absorption from ultraviolet to infrared region and photoluminescence (PL) emission at room-temperature. These phenomena are attributed to the Localized Surface Plasmon Resonance (LSPR) effect. Benefiting from absorption enhancement and surface heterojunctions, Ag@ZnO heterostructures show a photocurrent increment by 117%, a short response time of 80 ms and a recovery time of 3.27 s under 365 nm UV illumination of 0.24 mW/cm². This research presented a simple route to obtain high performance UV photodetectors and would be of some benefit in optical-electron devices manufacture.