Based on field survey, petrographic, ore microscopic and SEM/EDS analyses of ores and altered wall rocks as well as petrographic, microthermometric, LRM and LA-ICP-MS analyses of fluid inclusions in quartz, this paper deals with the characteristics and origin of ore-forming fluid and the metallogenic process in the Tianjingshan gold deposit. The results show that gold mineralization occurs dominantly in quartz veins and subordinately in altered wall rocks and is related mainly to sericitization and Fe-bearing carbonization and subordinately to chloritization, pyritization, chalcopyritization, galenitization and sphaleritization. Gold mineralization took place after the formation of quartz veins and took place during the brittle deformation stage of quartz. Native gold has been found together with sericite as infillings in quartz fractures or as cement of quartz breccia. Fluid inclusion analyses indicate that the ore-forming fluid is a water-rich and CO₂-bearing fluid characterized by middle-high temperature (286～380℃) and low salinity (3.0%～5.1% Nacl_eq). The LA-ICP-MS analyses of fluid inclusions in quartz reveal high Pb, Zn, Cu, Bi, Mo and Sb in the fluid. The H isotope measurements together with the enrichment of polymetallic components in ore-forming fluid imply an obvious magmatic origin. The upward migration of the magmatic fluid along the reactivated fault and the decreasing of temperature and pressure caused the boiling of the fluid. During the boiling process, the CO₂ preferentially went into the vapor phase and resulted in pH change of the ore-forming fluid. The change of pH caused the precipitation of gold and sulfides and also generated sericitization and Fe-carbonation.