The metal atom-supported catalysts have emerged as one of the most promising candidates for practical applications of photocatalytic hydrogen evolution (PHE) due to their maximized atom utilization efficiency. However, electronic modulation of the active species (single atoms, clusters or nanoparticles) on metal atom-supported catalysts remains a great challenge. In this work, the well-dispersed Pd single atom (Pd) and Pd sub-nanocluster (Pd) on TiO substrate (Pd/TiO) exhibit superb PHE activity with the hydrogen evolution rate (HER) of 53.64 mmol gh under simulated solar light irradiation and apparent quantum efficiency (AQE) of 40.16% at 365 nm. Theoretical calculations reveal that the synergistic effect of Pd and Pd can optimize the d-band center, and equilibrate the adsorption and desorption of intermediates during the reaction. In particular, Pd can effectively promote the electron transfer of Pd, resulting in excellent carrier separation efficiency of Pd/TiO and thus optimizing its catalytic activity.