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To maximize the photoelectrochemical (PEC) hydrogen production performance of quantum dot (QD)-decorated photoelectrodes, it is crucial to prioritize the optimization of electrode’s structure, including thickness and porosity. In this study, we prepare PbS QD-decorated mesoporous TiO₂ photoanodes for PEC hydrogen production, and systematically investigate the influence of the photoanode thickness on optical properties and PEC performances. As the thickness of photoanodes increases from 6.4 µm to 16.3 µm, the light absorption capability is enhanced across the entire visible and near-infrared (IR) spectrum due to the improved loading of PbS QDs. However, the photocurrent density is optimized for the 11.9 µm thick photoanode (15.19 mA/cm²), compared to the 6.4 µm thick (10.80 mA/cm²) and 16.3 µm thick photoanodes (11.93 mA/cm²). This optimization is attributed to the trade-off between the light absorption capability and the efficient mass transfer of the electrolyte as the photoanode thickness increases, which is confirmed by the lowest charge transfer resistance (<i>R</i>_ct) evaluated from the electrochemical impedance data.