In this study, a detailed model was proposed to simulate the process characteristic of flue gas desulfurization with multi-level humidifying in the underfeed circulating spouted bed (UCSB). The model conducted a steady state one-dimensional model by means of infinitesimal analysis, incorporating with the models of droplet evaporation, shrinking core and sulfur absorption. The trends of moisture, temperature, and sulfur concentration along the axial direction are calculated. The predictions were also compared with the experimental results, and they are in very good agreement. The results indicate: The inlet temperature and jet water flow rate are found to be very important factors to the sulfur removal efficiency. Applying multi-level humidifying, the outlet temperature decreases. As a result, the droplet evaporation gets slower and the sulfur removal efficiency increases. Degressive distribution of jet water is also found beneficial to acquiring higher desulfurization efficiency. (C) 2009 Elsevier B.V. All rights reserved.