The contamination of water by arsenic might result from oxidation of arsenopyrite. In this paper, the oxidation and dissolution process of arsenopyrite was run under the action of acidic surface water in the temperature range of 15~45℃by using the mixed-flow reactor. The results show that the oxidation process of arsenopyrite is speeded up with increasing concentration of FeCl3solution and rising temperature of reaction within the temperature range of 15~35℃, whereas the reaction rate of arsenopyrite is reduced at 45℃, and that the arsenic released from the reaction of arsenopyrite with FeCl3acidic solutions whose concentration is lower than 1×10-4mol·kg-1is in the form of As(Ⅲ), which is likely to be soon oxidized into As(Ⅴ) in the FeCl3acidic solution.