We first design and fabricate a highly efficient integrated CuWO4/Ag/BiOI device for photoelctrochemical water splitting. In this system, the formation of a p-n heterojunction between CuWO4/BiOI facilitates the carriers separation and transportation through built-in electrical field. Meanwhile, the Ag nanoparticles (NPs) own surface plasmon resonance (SPR), which increase the photoelectrochemical activity by transfer energy to metal oxides via hot electrons transfer (HET) and plasmon resonant energy transfer (PRET). As expected, this novel ternary CuWO4/Ag/BiOI structure reveals a dramatically promoted photocurrent of 2.10 mA/cm(2) at 1.23 V vs. RHE in photoelctrochemical water splitting, which is higher than that of bare CuWO4. Hence, this work may provide an avenue guidelines for p-n heterojunction and plasmon synergistically enhance photoelctrochemical water splitting.