The elaborate design and construction of a noble metal/semiconductor heterostructure have ignited tremendous interest in the photoelectrocatalytic (PEC) field thanks to the efficacious charge separation and high-reactive activity. Herein, ultrathin g-C3N4 nanosheets are decorated with Cul and Pt nanoparticles to construct a ternary composite photoelectrocatalyst for a direct methanol fuel cell via a facile approach. It is noted that the introduction of CuI, a typical hole transport channel, significantly improves the catalytic performance and long lifetime stability. More importantly, compared to dark conditions, the assistance of visible light remarkably promoted the current density, anti-poisoning ability, stability, and active area of Pt/g-C3N4/CuI. Efficacious electron transfer and high charge separation efficiency acquired through the formation of heterostructures and the photoelectric synergistic catalytic process are responsible for the significant improvement of photocatalytic performance. The present work provides a hopeful pathway to design a heterostructure catalyst with superior catalytic properties for applications in PEC fields.