The advantages of spray dry scrubbing of MSW incinerationflue gas include simple system, lowinvestment and operating costs, byproductwithout waste water and so on. Spray dry scrubbingis a very promising technique. Heat transfer, mass transfer and chemical reactionbetween alkaline dropletand flue gas in the spray dryer areanalyzed in detail. Calculation formulas of heat transfer and mass transfer of four-step of dropletdrying are gained. Influence factors of spray dry scrubbing acidgas componentsare also analyzed. The conclusionlay the foundation for establishing mathematicsmodel of spray dry scrubbingprocess.