Abstract: Located in the middle of Derbugan metallogenic belt in Inner Mongolia, the Badaguan copper-molybdenum deposit is a typical porphyry deposit with obvious phyllic and propylitic alteration zones. The banded orebodies occur in Triassic quartz diorite. By such means as Re-Os isotope geochronology, element geochemistry and sulfur isotope geochemistry, the authors preliminarily analyzed the ore-forming process of the deposit. Re-Os dating of molybdenite gave a weighted average model age of (226.7±2.4) Ma, implying that the deposit was formed in the early Late Triassic. Element geochemical analysis shows that the ore-bearing rocks were I-type granites and belonged to calc-alkaline rock series with moderate content of potassium, which were produced in a volcanic arc environment with characteristic active plate edges. During the ore-forming process, ΣREE in ore-bearing rock gradually decreased, and the ratio of LREE to HREE became higher. The δ34S values of metallic sulfides range from +0.5‰ to+4.8‰, suggesting mantle sulfur origin. Based on a comprehensive analysis, the authors hold that the Badaguan copper-molybdenum deposit was formed in the Late Triassic. During the southward subduction of Mongolia-Okhotsk Ocean plate, magmatic activity at the depth brought lots of ore-forming materials, and partially melted oceanic plate was emplaced above Ergun block. Cu, Mo and other elements migrated with the hydrothermal fluids and combined with sulfur from the mantle, eventually resulting in enrichment and mineralization.