The invention discloses a stable arsenic acid iron slag solidifying method. The stable arsenic acid iron slag solidifying method comprises the steps that arsenic acid iron slag, an additive and a reducing agent are mixed evenly, geierite is obtained through forming and reduction, and stable solidifying treatment is completed. The method can be used for directly producing geierite, dissolution of arsenic in waste slag can be effectively prevented, bounce does not occur with time, the geierite obtained after stable solidifying is high in strength, large in specific gravity and good in hydrolytic resistance, can be used for balance weight, the environmental environment caused by arsenic-containing slag stockpiling is eliminated, and stable solidifying of arsenic slag is achieved.