The surface mechanism of arsenopyrite, which was treated in different spans of time(0.5h, 2h, 4h, 6 h and 8h) in an alkaline environment(initial pH=12.3), was investigated by SEM, XPS and frontier orbital theory calculation. The results showed that Fe atoms were most easily oxidized and shifted onto arsenopyrite surface due to its activity. The dissolution of As atoms almost synchronized with that of Fe atoms, but S atoms were hardly suitable for this phenomenon. The oxidation degree of arsenopyrite surface increased with the treatment time. It is inferred that the reaction mechanism was that Fe atom on the arsenopyrite surface was firstly oxidized to ferric ion in alkaline solution, then As and S atoms were respectively converted into As(Ⅲ) and sulfate under the presence of ferric ion and oxygen, and As(Ⅲ) was finally absorbed onto the ferric hydroxide with porous structure. Combined with calcium ion, sulfate completely entered gypsum. As a result, these sediments could adhere to arsenopyrite surface.