Abstract: With the increasing prevalence of metabolic diseases caused by the modern dietary pattern, the “quality” rather than merely the “quantity” of carbohydrates has come under scrutiny for its impact on health. The Glycemic Index (GI), as a key indicator for evaluating the physiological effects of carbohydrates, has shifted from the intake amount to the post-meal blood sugar response, providing a theoretical basis for dietary management and prevention of chronic diseases including diabetes and obesity. The review first explains the definition, classification system, and standardized measurement methods of GI. Then, it reviews how the internal components of food (starch, protein, lipid, dietary fiber, and functional components) modify the digestion of starch and the physical structure of chyme, and how external processing techniques (thermal treatment, extrusion, enzymatic hydrolysis, and fermentation) physically and chemically modify the materials to regulate the release and absorption rate of carbohydrates, thereby determining the final GI value. Finally, it deeply explores the significance of GI in regulating post-meal blood sugar fluctuations, which affects metabolic health, cardiovascular risk, and weight management. The review not only deepens our understanding of the food’s glycemic effect but also lays an important foundation for developing low-GI foods and achieving precise nutritional intervention.