Abstract: The Yiermande deposit represents a type of gold deposits in Western Tianshan.The ore-forming fluid in Yiermande gold deposit can be divided into four stages.Study on the fluid inclusions shows that the ionic component in fluid is mainly K⁺,with minor Na⁺, Cl^- and SO₄^2-, and less Ca²⁺ and Mg²⁺.The gas phase is composed of mainly H₂O, with minor CO₂, H₂, O₂, N₂, CH₄ and CO. The ore-forming fluid exhibits 8.28 g/L in average degree of mineralization,with pH value ranging from 2.8 to 4.6, salinity from 1.5% to 0.35% (equal NaCl), ionic strength from 0.146 to 0.18, and lg_fO₂ value from -35.2 to -38.9. The stable area of lgf O₂-pH of Yiermande gold deposit is different from both that of the adularia-silicate type and that of acidic sulfate type.Its stable area of lgf O₂-pH is just in between the other two types.The mean temperature of fluid is from 93 to 151.4℃,while the pressure from 5.6 to 17 MPa.The fluid belongs to H₂O-NaCl-CO₂ system,mainly derived from meteoric water. The Yiermande gold deposit is supposed to be epithermal deposit.