Abstract: Thermodynamic problem is the core issue of the sulfuric acid leaching reactions of vanadium bearing steel slag.According to the mineral compositions in vanadium bearing steel slag,thermodynamic calculation and analysis of the steel slag leaching system were carried out in this paper.The results show that,tricalcium silicate can react with sulfuric acid easily as a result of low and high equilibrium constant of acid dissolution reactions of tricalcium silicate at room temperature.Therefore,much of sulfuric acid will certainly be consumed due to the high content of tricalcium silicate in the steel slag.FeO can be fully leached by sulfuric acid in theory while vanadium cannot be leached at the terminal acid concentration（leaching equilibrium acid concentration） of 10^-4 mol/L,which indicates that the thermodynamic trend of FeO leaching is greater than that of vanadium leaching.Based on this result,a new leaching method of "selective stage leaching" can be designed,and a better selective leaching separation between vanadium and iron can be achieved significantly with pre-leaching of impurities in the first stage and leaching vanadium in the second stage.Eh-pH diagram shows that VO²⁺,VO²⁺,and V³⁺ are the main dissolved phases of vanadium in the acidic media,and the stable region of VO²⁺ is very large and just in the middle of the three dissolved phases.Thus in the leaching process,the vanadium should be controlled in the stable region of VO²⁺.