The Diyanqin'amu Mo deposit is located in Dong Ujimqin Banner, Inner Mongolia. The barely exposed intrusive rocks in this deposit are mainly composed of fine-grained syenite and mafic dikes. The Mo mineralization predominantly occurs in Jurassic volcanic rocks with minor in fine-grained syenite veins, mainly including veinlet, vein and disseminated mineralization types. Ore-bearing quartz-potassium feldspar-fluorite miarolitic cavities or hydrothermal veins with these minerals in the center are identified in the fine-grained syenite, which are direct evidence for exsolution of magmatic fluids. In this study, based on detailed petrographic observation, we conducted cathodoluminescence imaging for main minerals in the quartz-potassium feldspar-fluorite miarolitic cavities, microthermometry for fluid inclusions in quartz and fluorite, and compositional analysis for fluid inclusions using SEM/EDS, LRM and LA-ICP-MS. On the basis of above analyses we discussed the process of fluid exsolution, features of initial ore-forming fluids and precipitation mechanism of ore minerals. Our results show that the Mo mineralization was genetically related to the magmatic fluids derived from fluid exsolution of the fine-grained syenite. Enriched fluorine and other volatiles decreased the solidus and viscosity of the ore-related magma, which enabled rapid upwelling of this melt to the shallow upper crust via faults. Rapid decompression during ascent was probably the major mechanism for fluid exsolution of magma. The initial magmatic fluids were F- and CO₂-riched fluids with medium-high temperature (227~457℃) and medium-low salinity [0.3%~8.6%, w(NaCl_eq)]. The initial magmatic fluids separated into a CO₂-riched gas phase with low salinity and a liquid phase with high salinity owing to boiling caused by rapid decompression. Therefore, the escape of CO₂ caused by boiling was probably the main reason for molybdenite precipitation.