A thermodynamics analysis on the leaching process of selenium residue and discussion on the behaviors of aqueous ionic in the leaching process were carried out.Through thermodynamical calculation,the values of △G(-)T and relevant potential expressions were obtained.According to these thermodynamical data,the Φ pH diagrams of Se-H2O system at 298 and 373 K were obtained; Simultaneously,the Φ-pH diagrams of SO2-H2O and SO2-Se-H2O systems were drawn at 298 K.With increasing the temperature,the stable regions of HSeO3-,SeO32 and SeO42 in the Φ pH diagram of Se-H2O system become gradually large,but the limits of pH in the stable region become gradually small.The stability area of reduction precipitation in the SO2-Se-H2O system was finally determined.The results of oxidization leaching experiments of selenium residue indicate that when the mass ratio of selenium residue to sodium chlorate is 2,the concentration of sulfuric acid is 300 g/L and the residue is agitated for 3 h at leaching temperature of 80 ℃,the leaching rate of selenium could reach 97.76 ％.The experimental results conform the calculated results by Φ-pH diagram.The selenium reduction precipitation in oxidization-leaching solution was analyzed under the conditions of acidity of 150 g/L,the sodium sulphite concentration of 35 g/L at the reductive temperature of 23 ℃ for 120 min.And this demonstrates the thermodynamics analysis.