Arsenic sulfide slag (ASS) is typically by-produced from arsenic-containing wastewater treatment. In this work, a novel hydrothermal treatment method with the assistance of Fe(NO3)3 (HT-Fe(NO3)3) was developed to detoxify ASS by transforming arsenic into scorodite and extracting sulfur in one step. After hydrothermal treatment, As(III) in ASS was oxidized and immobilized into the stable scorodite with a high As immobilization efficiency (~99%), and the toxicity leachability of arsenic-containing solid waste significantly reduced from 634.2 to 2.5 mg/L, well below the discharge standard of solid waste. Further study reveals that the nucleation and growth process was fit well by Avrami-Erofeev model and followed Ostwald step rule, which involved the As2S3 dissolution, formation of amorphous ferric arsenate and then crystallization within the amorphous precursor. In this process, sulfur originated from As2S3 played an important role by serving as the heterogeneous nuclei to decrease the barrier for the formation of amorphous ferric arsenate, and facilitated the transformation of as-formed scorodite from nano-sheet aggregates to the bulk and dense spherical polymorph, which further increased the stability of the arsenic contained solid product. This study will shed light on the development of new technologies for treatment of industrial solid waste and recycle of useful resources.