Arsenic existed concomitantly with minerals containing precious or non-ferrous metals, in which arsenic-containing wastewater (ACW) is inevitably generated during the process of smelting. In the treatment of ACW, the precipitation method using sulphide is widely used, but the resulting arsenic sulphide slag poses a risk of secondary pollution. In this work, a reduction strategy was developed to transform arsenic sulphide slag into elementary arsenic under hydrothermal conditions by using sodium formate as the reductant, with the aim to treat the slag and recycle As. The experimental results show that As(0) can be obtained by one-step hydrothermal reduction by sodium formate at temperature of 200 ℃ and pH 4, and the purity of elementary arsenic produced under the optimum conditions is higher than 90%. Mechanism analyses reveal that the reduction process of As₂S₃ to As(0) experiences two steps, where amorphous As₂S₃ primary particles first transform and crystallize into partially reductive AsS in a bulk size, and then AsS is further reduced into As(0) through a collapse process to form small particles. This work provides a recycling strategy for the efficient conversion of arsenic sulphide slag to As(0) for potential applications.