Abstract: The occurrence of As in coal and its behavior during coal combustion is reviewed .The enrichment mechanism of As was discussed in view of the evolution of fly ash.It was concluded that the evolution of fly ash and the enrichment of As on fly ash are two respects of the same process.As reacted with fly ash and produced stable arsenic compounds,coming into crystal lattice of clay minerals,absorbed by fly ash and condensed on the surface of fly ash.It is considered to be the four main forms of the enrichment of As by fly ash.