Polyoxometalates possess unique electron-accepting ability while maintaining their structures; this makes them effective electron mediators in semiconductor-based optoelectronic system via retarding the photoexcited electron-hole recombination. H3PW12O40 (PW12) was introduced into TiO2 film using layer by-layer method in the presence of reduced graphene oxide, serving as a conductive additive. The composite film had smooth surface and dense packing of TiO2 nanoparticles, and showed enhanced transient photocurrent compared to the films without PW12 or reduced graphene oxide (rGO). I-V measurement further confirmed that the power conversion efficiency of PW12/rGO/TiO2 film was remarkably improved compared to those of the films without PW12 or rGO. PW12 functions as an electron mediator and rGO serves as a conductive agent for enhancing the photocurrent and power conversion efficiency, due to the efficient dissociation of excitons and fast electron transfer.