Based on the theory and method of thermodynamics, this paper discusses the effect of CO<SUB>2-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=707.946 bar), acid(pH=3.71), reducing(fO 2=5.012×10-36 bar), mid-temperature(267 ℃) and overpressure(1 800 bar) gold-bearing fluid(a∑Au=3.744×10-8 mol/L). This led to the solubility of Au in ore-forming hydrothermal solution(a∑Au=3.790×10-9 mol/L), rapid precipitation of Au, and the formation of ore deposit.