In a wet desulfurization process NaOH solution is sprayed into flue duct, small droplets are then formed and react directly with gaseous SO2 in FG. In this paper a two-phase mathematical model based on dual film mass transfer theory and boundary layer theory on the surface of droplet is proposed to describe the reacting processes between the gaseous SO2 and a single droplet Then according to the droplet diameter distribution function, a set of formulas to estimate the binding capability of droplets on SO2 and to predict the desulfurization efficiency are established. Through numerical calculation, key factors such as pH value of droplets, the ratio of spraying NaOH solution volume to the volume of FG, etc. are analyzed and discussed. Additionally, a SO2 removal pilot experiment is carried out on the flue duct of a coal-fired industrial boiler with steam capacity of 10T/h to test the SO2 removal rate.