Abstract: In recent years, soymilk (traditional soymilk and fermented soymeal) root irrigation has become a routine agricultural technical measure in Sanmenxia tobacco-planting areas of Henan Province. To unravel the effects of soymilk root irrigation on the quality of tobacco and related mechanisms, pot and field experiments were conducted to analyze the effects of soymilk root irrigation on tobacco root growth, agronomic traits and soil biochemical characteristics (enzyme activity, active carbon and soil protein index). The results showed that soymilk root irrigations improved the quality of tobacco including physical characteristics, chemical composition, appearance and sensory property. Soymilk root application also increased the middle leaf length, upper leaf width, root surface area, root volume and root dry weight. Soil enzyme activities changed little after soymilk irrigation; however, soil active carbon increased significantly at the early stage after irrigation, soil protein index could maintain a high level in a long time after soymilk root irrigation. Therefore, the enhancement of soil protein index and active carbon content after soymilk root irrigation may be one of the main reasons for its promotion of tobacco root growth and tobacco leaf quality.