Abstract: Bismuth oxide was used for dechlorination from wastewater in zinc smelting, but the detailed dechlorination mechanism and boundary conditions are still unclear. Based on thermodynamic calculation, the phase and morphology transitions of bismuth oxide in the whole process included acidification-adsorption dechlorination - alkaline dechlorination - regeneration cycle were investigated. The thermodynamic calculations show that, in Bi³⁺-Cl-H₂O system, Cl can be removed by forming the precipitation of BiOCl in the pH range of 0~10. With the increase of pH, the main phase of Bi was transformed from Bi³⁺ to BiOCl and Bi₂O₃ in turn. When n(Bi)∶n(Cl)=1∶1, the lowest Cl concentration in the solution was 3.91 mg/L, and the presence of SO₄^2- had no effect on the chlorine distribution. Experimental studies show that bismuth oxide was changed to bismuth sulfate and basic bismuth sulfate respectively below and above 60 g/L sulfuric acid when acidified with sulfuric acid, and the morphology changed from cluster to regular rod. Both of them can adsorbed with Cl in acid solution to form BiOCl precipitation. The BiOCl can be regenerated into Bi₂O₃ after NaOH alkali washing and returned to utilization. After 10 cycles of bismuth oxide, the Cl removal rate was still higher than 90%, and the chlorination removal stability was good.