To extract vanadium efficiently from the acidic leachate of vanadium slag, a thermodynamic analysis of phosphorus-sulfur-vanadium-water acidic systems was established at 298 K. Results show that in the P(V)-V(V)-H₂O acidic system, VO₂⁺ is first conver-ted to phosphovanadic heteropolyacid ions when pH=1~4 and then converted to vanadium isopolyacid ions when pH=4~7. In the S(VI)-V(V)-H₂O acidic system, VO₂⁺ and VO₂SO₄^- are the main existing chemical forms when pH=0~1 and gradually converted to vanadium isopolyacid ions when pH=2~6. In the P(V)-S(VI)-V(V)-H₂O acidic system, when pH=1~3, VO₂⁺ and VO₂SO₄^- are gradua-lly converted to phosphovanadic heteropolyacid ions. The optimum molar fraction of ΣPV₁₄ (total sum of phosphovanadic heteropolyacid ions) reaches 88.55% at pH=2. As the pH value increases to 4~6, phosphovanadic heteropolyacid ion gradually disappears and is converted to vanadium isopolyacid anions, and the optimum molar fraction of ΣV₁₀ (total sum of vanadium isopolyacid ions containing ten vanadium) is 100.00% at pH=5.