In order to evaluate the water retention of tobacco quickly and accurately, a model was developed based on water activity. Gibbs free energy change (GFEC) of tobacco was calculated according to thermodynamic correlation formula, and then the moisture migration of tobacco was predicted on the basis of GFEC. For a multi-component system, the GFECs of the variable component, invariable components and the whole sample were calculated separately to investigate the moisture variation of the cigarette blend in a test environment and the effects of different component proportions on water loss or absorption. The model was calibrated and checked by water detection and scanning electron microscopy. The results showed that: 1) GFEC could be used as a criterion of water loss or absorption of tobacco. 2) For the sample of cigarettes, the spontaneous reaction at room temperature is water loss if its GFEC is less than zero. The lower the GFEC is and the higher water dissipation rate from tobacco is. When GFEC is close to zero, water in tobacco remained broadly unchanged. When GFEC is positive, tobacco absorbs water, and the absorbing rate increases as the GFEC increases. 3) GFEC model is adaptive for single component systems, multi-component systems, variable systems and solid-liquid mixing systems. The results of tobacco blending experiments at different proportions and type of humectant validated the accuracy of the GFEC model. The model has the advantages of being simple and rapid; it is suitable for evaluating the water retention of tobacco blends.