On the basis of extensive survey to the Quaternary paleosols, soils and weathering layers in the vast northern China and the Loess Plateau, we found some unusually special phenomena of chemical components such as unsuccessive illuvium, twin illuviums, unusually thick illuvium and multi-illuviums etc. According to the analysis on the content of CaCO 3 and the data of penetrating experiment, a new theory of illuvial depth or removal depth of CaCO 3 in weathering and leaching belt was constructed, which indicates that the time of CaCO 3 removing to the illuvial depth is very short, and the effect of time factor on illuvial depth can be ignored. The theory can be taken as a credible foundation for studying many geological and geographical problems in weathering and leaching belt. When the illuvial depth of CaCO 3 is bigger than the thickness of developing belt of soil or paleosol, it can be determined that the paleosol has turned into weathering crust. When the illuvial depth of CaCO 3 is bigger than the thickness of paleosol, paleosol is leaching moderately acid soil. When two, three layers or unusually thick CaCO 3 illuviums exist in the same weathering section or at the bottom of the same paleosol, there were two or more periods forming paleosol and corresponding climatic stages at that time. On the basis of the equation of relationship between mean annual rainfall (y) and illuvial depth of CaCO 3 (x) (y = 305.5x + 168.5) determined in the paper, mean annual precipitation during the development of paleosol can be calculated. On the basis of extensive survey to the Quaternary paleosols, soils and weathering layers in the vast northern China and the Loess Plateau, we found some unusually special phenomena of chemical components like as unsuccessive illuvium, twin illuviums, unusually thick illuvium and multi-illuviums etc . According to the analysis on the content of CaCO3 and the data of penetrating experiment, a new theory of illuvial depth or removal depth of CaCO3 in weathering and leaching belt was constructed, which indicates that the time of CaCO3 removal to the illuvial depth is very short, and the effect of time factor on illuvial depth can be ignored. The theory can be taken as a credible foundation for studying many geological and geographical problems in weathering and leaching belt. When the illuvial depth of CaCO3 is bigger than the thickness of developing belt of soil or paleosol, it can be determined that the paleosol has turned into weathering crust. When the illuvial depth of CaCO 3 is bigger two than three layers of paleosol, pale two layers of paleosol climatic stages at that time. On the basis of the equation of relationship between mean annual rainfall (y) and illuvial depth of CaCO 3 (x) (y = 305.5x + 168.5) determined in the paper, mean annual precipitation during the development of paleosol can be calculated.