The Longtoushan polymetallic sulfide deposit is located on the east slope of the middle-south Da Hinggan Mountains and the north side of the Xar Moron River fault along the Hercynian geosyncline fold belt. Field geological survey shows that the ore deposit has two mineralization types, namely layered mineralization and cylinder mineralization. Attitudes, alterations and minerals of the two mineralizeation types show typical characteristics of submarine hydrothermal sedimentary deposition. A systematic study of fluid inclusion petrography and microthermometry was carried out for fluid inclusions in syndepositional barite from the ore deposit. The inclusions in the barite suggest rapid sedimentary characteristics during the hydrothermal sedimentary deposition. Except for secondary fluid inclusions, gas-pure and gas-rich inclusions related to the mineralization are very rare, with the majority being liquid-rich inclusions. Microthermometric analysis shows that the liquid-rich inclusions have homogenization temperature 101.4～279.9℃, freezing temperature -5.5～-0.3℃, salinity 0.5% ～8.5%, and fluid density 0.75～0.97 g/cm³. Reflective features such as temperature, salinity and density of the ore fluids are consistent with features of the inclusions developed in submarine hydrothermal sedimentary sulfides ore deposits. The results provide positive evidence for the viewpoint that the Longtoushan polymetallic sulfide ore deposit is a marine hydrothermal sedimentation deposit.