The waste acid in acid plant industry by smelting flue gas usually contains a certain amount of metal ions and arsenic ions with higher toxicity. The sulfidation method can remove Cu2+ and As3+ therein step by step. In this paper, theoretical calculations and analysis of the process were carried oud, in which step-by-step sulfidation is conducted towards the metal ions in acidic wastewater containing copper and arsenic with an acidity of 0-15%. At the same time, theoretical calculations were also carried out on the pH range of the acid waste water with the addition of a sulfiding agent to avoid the generation of sulfur. The relevant confirmatory experiments proved that the theoretical calculation results are in good agreement with the experimental results. The acidity of the waste water(0-15%) has little effect on the removal of copper ions in first sulfidation. When the amount of sulfiding agent in first sulfidation reaches 1.0 times the theoretical demand for removing copper ions, the removal rate of copper ions reaches over 99%. In second sulfidation, when the amount of sulfiding agent reaches 1.0 times the theoretical demand for removing arsenic ions, the removal rate of arsenic ions reaches over 96%.