The photochemical system, which utilizes only solar energy and H₂O/CO₂ to produce hydrogen/carbon-based fuels, is considered a promising approach to reduce CO₂ emissions and achieve the goal of carbon neutrality. To date, numerous photochemical systems have been developed to obtain a viable solar-to-fuel production system with sufficient energy efficiency. However, more effort is still needed to meet the requirements of industrial implementation. In this review, we systematically discuss a typical photochemical system for solar-to-fuel production, from classical theories and fundamental mechanisms to raw material selection, reaction condition optimization, and unit device/system advancement, from the viewpoint of ordered energy conversion. State-of-the-art photochemical systems, including photocatalytic, photovoltaic-electrochemical, photoelectrochemical, solar thermochemical, and other emerging systems, are summarized. We highlight the existing bottlenecks and discuss the developing trend of this technology. Finally, optimization strategies and new opportunities are proposed to enhance photochemical systems with higher energy efficiency.