Abstract: Located in the Sankuanggou-Duobaoshan tectonic-metallogenic belt of Da Hinggan Mountains, the Sankuanggou Cu-Fe-Mo polymetallic deposit is a typical skarn deposit. Petrographic observation and microthermometric study were carried out for fluid inclusions from garnet at the anhydrous skarn stage (Ⅰ), quartz at the hydrous skarn-oxide stage (Ⅱ), quartz at the early sulfide stage (Ⅲ) and calcite at the early late sulfide stage (Ⅳ). Petrographic observations reveal that various types of fluid inclusions are present in hydrothermal minerals of different ore-forming stages, comprising mainly vapor-liquid two-phase inclusions, subordinately vapor inclusions, and occasionally liquid inclusions as well as NaCl daughter mineral-bearing multiphase inclusions in quartz. Studies show that their homogenization temperatures vary from 152℃ to 478℃, their salinities are 1.57~58.02 wt% NaCl, and their densities range from 0.64 g/cm3 to 1.18 g/cm3, thus belonging to the ore-forming system of medium-high temperature, medium-high salinity and medium density. In addition, mineralization pressures were calculated on the basis of homogenization temperatures and salinities, and the results show that the mineralization pressures vary from 39.44 MPa to 133.65 MPa, whereas the ore-forming depths are in the range of 3.94 km to 9.64 km, indicating that this ore deposit was formed in a mesogene ore-forming environment.