Through the establishment of isotherm moisture absorption curve model equation in the range of water activity 0.1-0.9 for cut tobacco (cut leaf, expanded tobacco, stem, air flow tobacco and cut tobacco), the prediction model between water activity and water content of cut tobacco was established by the method of benchmarking oven, and further validation was made on the basis of the model equation for the humidity range of 40%-80% in tobacco processing environment. The parameters of the model equation are modified by the results, and then the actual samples are tested. The results show that the determinant coefficient of the model equation is better than 0.999 8, and the mean relative error of the moisture content of cut tobacco predicted by water activity is less than 3%. The maximum difference between the predicted and measured moisture content was 0.46%.