<p>Photocatalytic technology holds significant promise for addressing global energy and environmental challenges. The emergence of the high-low junction represents a pivotal breakthrough in this field. This review comprehensively examines the concept, mechanism, fabrication strategies, diverse photocatalytic applications, and advanced characterization techniques of high-low junctions. Derived from S-scheme heterojunction and p-n junction principles and optimized beyond conventional type-I heterojunctions, high-low junctions enable efficient separation/migration of photogenerated carriers while retaining highly redox-capable species, thereby significantly enhancing photocatalytic performance. We focus on key fabrication strategies and elucidate their respective advantages and limitations in constructing heterojunctions with tailored energy band structures. The review further highlights the exceptional performance of these junctions in multiple aspects of applications. By systematically categorizing advanced characterization and theoretical simulation techniques according to their functionalities, this review provides multi-scale analysis of interfacial charge dynamics and atomic-level mechanistic insights. These insights offer crucial guiding principles for material design and photocatalytic mechanism interpretation. Finally, we discuss emerging research frontiers centered on interface engineering, advanced characterization, and machine learning-guided discovery to address persistent challenges in carrier recombination and stability optimization. It is expected that this comprehensive analysis will provide a foundational reference and inspire future innovations in high-low junction photocatalysts with novel charge transfer mechanisms.</p>