The invention relates to a method for predicting the water activity of a mixed system of tobaccos and tobacco products as well as the moisture content of each component. The method is characterized by comprising the steps of drawing a moisture-absorption isothermal working curve of each component of the tobaccos and the tobacco products before mixing, acquiring relevant parameters of a moisture-absorption isothermal working curve equation of each component by virtue of a DLP (double log polynomial) model, calculating parameters of the DLP model of the mixed system according to the dry weight proportion of each component, establishing a moisture-absorption isothermal working curve equation of the mixed system, and predicting the equilibrium water activity of the mixed system and the equilibrium moisture content of each component of the mixed system. The method has the beneficial effects that the method for predicting the water activity of the mixed system of the tobaccos and the tobacco products as well as the moisture content of each component is provided for the first time, and the moisture control during the product design of tobacco enterprises can be guided, so that the blindness on screening of each component of raw materials, auxiliary materials and packaging materials of the tobaccos can be reduced.