Since the beginning of the 21st century, energy shortage and environmental pollution have been the major challenges faced by human beings. Photocatalytic carbon dioxide (CO₂) reduction is one of the promising strategies to solve the energy crisis and promote the carbon cycle, in which semiconductor captures solar energy to obtain hydrocarbon fuel. However, the low activity and poor selectivity of the products greatly limit the practical application of this technology. Thus, it is of great significance to regulate product selectivity, improve photocatalytic efficiency, and deeply understand the mechanism of CO₂ reduction reaction. In recent years, ultrathin materials have attracted extensive attention from researchers due to their high specific surface area, abundant unsaturated coordination surface atoms, shortened charge migration path from inside to surface, and tailorable energy band structure, and have achieved promising results in the field of photocatalytic CO₂ reduction. In this paper, the reaction mechanism of photocatalytic CO₂ reduction is firstly summarized. Next, the research results of promoting electron hole separation and regulating charge transport path of ultrathin nanostructures by constructing heterostructures, designing Z-scheme systems, introducing co-catalysts, and defect engineering are introduced. Finally, the prospect and challenge of improving the efficiency of photocatalytic CO₂ reduction and optimizing the product selectivity are pointed out.