The photoelectrochemical (PEC) properties of BiVO4 are limited by the poor separation and transfer of photogenerated electron-hole pairs and easy recombination. Hence, we designed the ternary composite photoelectrode through the embedding of Ag in BiVO4/Cu2O p-n heterojunction and evaluated its PEC performance by degrading Rhodamine B (RhB) for the first time. As expected, the BiVO4/Ag/Cu2O photoelectrode among four of the BiVO4-based photoelectrodes achieved the highest photocurrent density (3.36 mA/cm2), which is about 8.6 times than pure BiVO4 photoelectrode (0.39 mA/cm2). Additionally, the resulting fabricated BiVO4/Ag/Cu2O composite as the working photoelectrode performs a highly efficient degradation of RhB molecules in water solution through PEC process (86% within 120 min), which is 2.8 times than pure BiVO4. It is noteworthy that the generation of built-in potential at the interface of BiVO4/Cu2O p-n heterojunction plays a crucial role in the carriers' separation efficiency. Meanwhile, the surface plasmon resonance (SPR) of Ag nanoparticles (NPs) displays a magnificent role in improving the PEC performance by hot-electrons injection (HET) and plasma resonance energy transfer (PRET) to metal semiconductors. This research may inspire a new design for the synergistic effect of p-n heterojunction and plasmon to enhance PEC properties of pollution degradation.