In order to achieve harmless disposal of high-arsenic waste acid and efficient arsenic solidification, the idea of using solid iron sources for arsenic removal from waste acid was proposed, and the influence of solid iron oxide on the arsenic removal process from high-arsenic waste acid and the synthesis of scorodite was explored. The results showed that the full reaction between solid iron oxide and waste acid at room temperature could provide initial iron ions for the arsenic removal reaction and reduces the pH of the solution. This reaction was conducive to the initiation of arsenate precipitation reaction, and aided in the stable nucleation and good crystallization of scorodite. During the experiment, it was found that the presence of arsenate ions can promote the dissolution of solid iron oxides in the system, and it was revealed that the mutual promotion effect of "solid iron oxide dissolution-arsenate precipitation" existing in an acidic environment was an important way to achieve efficient arsenic removal. Fe3O4 magnetic nanoparticles (MNPs) underwent Fenton reaction during the arsenic removal process, producing abundant hydroxyl radicals. This reaction can promote the dissolution of Fe3O4 in the solution and accelerated the precipitation of arsenate, giving the Fe3O4 the best arsenic removal performance. When the reaction time was 12 h, the pH was 1.5, the Fe/As molar ratio was 2, and the reaction temperature was 90℃, Fe3O4 had the best arsenic removal efficiency in waste acid, which could reach 99.93%. Based on this, a new process was proposed for the efficient treatment of contaminated water using Fe3O4, including efficient arsenic removal of high-arsenic contaminated acid and further purification of low-arsenic contaminated acid. This process has a high arsenic removal rate while avoiding secondary pollution. By using scorodite to remove arsenic, the adsorption and magnetic characteristics of MNPs were used to achieve deep removal and efficient solidification of arsenic in waste acid.