Abstract: High-precision focus detection technology is a core key technology in optical systems such as lithography, microscopic imaging, precision optical inspection, and machine vision, directly determining system image clarity, processing accuracy, and measurement stability. Addressing the limitations of traditional focus detection methods—including restricted detection accuracy, weak anti-interference capability, slow dynamic response and poor adaptability to large fields of view and complex operating conditions, academia and industry both at home and abroad have continuously iterated and optimized focus detection principles, image sharpness evaluation functions and hardware system designs. In this paper, the working principles of mainstream focus detection techniques, including off-axis triangulation, the critical angle method, and aberration-based methods, were systematically explained. The focus detection solutions adopted by leading international lithography companies were reviewed, and recent research progress in domestic focus detection technologies was analyzed. Finally, the current state of the industry was summarized, several potential optimization directions were identified, and the future development and diversified applications of focus detection technology were discussed.