Abstract: A polymetallic deposit zone with W, Mo, Cu, Pb, Zn mineralization was found in the western part of the North Qilian Mountains. The Xiliugou polymetallic deposit is located in the western part of this polymetallic deposits zone. In order to discuss the source of ore-forming fluid of this deposit, the authors selected 9 ore samples of the main ore-forming stage from this deposit and measured their H, O, C isotopic compositions. The results show that the DV-SMOW‰ values of the ore-forming fluid are -47‰ ~-72‰ with very small variation. The δ18O values in the main stage vary between 11.9‰ ~14.1‰. The δ18OH2O‰ values of the hydrothermal fluid equilibrated with quartz are -0.79‰ ~3.64‰（ the average homogenization temperature of the fluid inclusions is 190℃）. The δ13C values of fluid inclusions captured in the quartz are -6‰~-4.2‰, also with a small variation range. Microthermometric studies indicate that the homogenization temperatures and salinities of the fluid inclusions in the oreforming stage range from 126 to 384℃ and from 1.05% to 16.45% NaCl equivalent, respectively, with most values in the range of 140~200℃ and 1.74%~6.45% NaCl equivalent, respectively. It is shown that the Xiliugou polymetallic deposit is a shallow, low temperature, low salinity and low density hydrothermal deposit. The magmatic water is the main source of the ore-forming fluid, and the meteoric water is subsidiary.