Achieving high photocatalytic activity with the lowest possible platinum (Pt) consumption is crucial for reducing the cost of Pt-based cocatalysts and enabling large-scale applications. Bimetallic Ni-Pt cocatalysts exhibit excellent photocatalytic performance and are considered as one of the most promising photocatalysts capable of replacing pure Pt for the hydrogen evolution reaction (HER). However, the synergistic photocatalytic mechanism between bimetallic Ni-Pt cocatalysts needs to be further investigated. Herein, we deposit highly dispersed Ni-Pt bimetallic cocatalysts on the surface of TiO2 by radiolytic reduction. We study the dynamics of photogenerated charge carriers of the Ni-Pt comodified TiO2 and propose their underlying electron transfer mechanisms, in which Pt acts as an electron trap, whereas Ni serves as an electron supplier. The synergistic effect is Ni/Pt ratio-dependant, and can confer bimetallic Ni-Pt to pure Pt-like photocatalytic activity in HER. The Ni2-Pt1 co-modified TiO2 is optimized to be the most cost-effective photocatalyst with robust stability, which outperforms about 40-fold higher performance than bare TiO2.