Abstract: The leaching characteristics of arsenic in fly ash from coal combustion in lab-scale fluidized bed were clarified. Calcium oxide was mixed with fed coal for matching different Ca/S molar ratio to investigate the role of calcium on the leaching behavior of arsenic. The results indicate that increase in Ca/S molar ratio promotes the retention of arsenic in fly ash and in turn reduces its emission. The reaction between CaO and arsenic is largely dependent on the sulfation of CaO. Leaching of arsenic from fly ash is greatly dependent on the pH value of leachate. Alkali fly ash generates a high pH value solution during leaching test, facilitating the prohibition of arsenic leaching. The mechanism of arsenic leaching in alkali fly ash can be described as that arsenic first releases from fly ash quickly and then subjects to the reaction with calcium to form precipitate under high pH value condition.