Abstract: This study aimed at the stabilization of high As -contained caustic sludge taken from an antimony smelter in Hunan Province. The influences of Ca/As molar ratio, liquid/solid mass ratio, temperature and reaction time on the effects of calcium -arsenic crystallization were investigated. The results show that when the Ca/As molar ratio is 3.5 :1, the liquid/solid mass ratio is 3 :1, the temperature was 40 ℃ and the reaction time is 8 h, the As leaching concentration is the lowest descending from 1 348.67 mg/L to 0.13 mg/L which is better than standard values in GB 5085.3-2007. Besides, the XRD patterns and the SEM micrographs reveals that the amorphous Ca-As compounds in original slag transforms into stable well-crystallized calcium arsenate hyroxide hydrate (Ca₄(OH)₂(AsO₄)₂·4H₂O) after crystallization .