Abstract: Fluid inclusions sampled from mother lodes of Xin'anzi quartz type W-Sn deposit were examined by petrography, microthermometry and Raman microprobe. The results show t hat: ① the homogeneous temperatures of fluid inclusions in the quartz distribute intensively in three intervals, i.e. 320 ℃～360 ℃, 260 ℃～300 ℃ and 180 ℃ ～230 ℃ , which might correspond to the three stages of the whole ore -forming process. Wolframite is formed mainly in the high-temperature stage while sulfides in the medium-temperature stage; ② The process that mixing of magmatic water with meteoric water and its companion boiling are probably not the key mechanism of W-Sn and sulfides mineralization because they have occurred in the late stage of mineralization (180 ℃～230 ℃); ③The gaseous phase of the fluid inclusions both in quartz and in topaz are composed of H₂O and a small amount of CO₂ and CH₄. With the decrease of the homogeneous temperatures, the content of CO₂ tends to degrade while that of CH₄ is the opposite. It indicates that the fluid transformed gradually from an oxidizing solution to a reducing one, which might be the main mechanism which lead to the sulfide mineralization.