Abstract: Based on the theory and method of thermodynamics，this paper discusses the effect of CO2-H2O fluid immiscibility on the solubility of Au. The results indicate that the ore-forming fluid in the Shuiyindong gold deposit was a kind of volatiles-rich( fCO 2=707.946bar)，acid (pH=3.71)，reducing (fO 2=5.012×10-36bar)，mid-temperature (267℃) and overpressure (1800bar) gold-bearing fluid (a∑Au=3.744×10-8mol/L). When the closed layer of overpressure fluid was destroyed by the fault, pressure suddenly decreased (285～353bar), CO2-H2O fluid immiscibility occurred, and a large amount of CO2 overflowed. The fugacity of CO2 and O2 decreased (fCO 2=8.054bar，fO 2=2.512×10-41bar), pH increased (pH=4.32), and temperature decreased too (224℃) with the fluid immiscibility. This led to the solubility of Au in ore-forming hydrothermal solution(a∑Au=3.790×10-9mol/L), rapid precipitation of Au, and the formation of ore deposit.