Photoelectrochemical water splitting is a promising approach to produce green and renewable hydrogen fuel, alleviating the CO2 emissions, air pollution, and energy crisis. However, the efficiency is limited by the recombination of photogenerated carriers and the losses of holes, resulting in a mismatch between the rates of water oxidation and reduction reactions. This article starts with a discussion of the principle of photoelectrochemical water splitting, highlighting the role and importance of holes, and then summarizes the development of the hole transport layer, with a focus on the classification of the hole transport layer, the structure and properties of common hole transport materials, and the construction and improvement of the hole transport layer. Finally, it is concluded with a summary and perspective of strategies for the future development of the hole transport layer.