Based on density functional theory (DFT), the adsorption properties of arsenic (As) via the five most abundant oxides in fly ash were investigated. It has been proved that the iron oxide (Fe2O3) in fly ash is the effective active component during the adsorption process. This paper verifies the feasibility of using fly ash as the adsorbent to control As pollution in power plants, and also provides some reference for the subsequent experimental research.