Abstract: Through the detailed observation of the geological characteristics of Yuanzhuding porphyry Cu-Mo deposit，and combining with the tests on the inclusions at each stage and H，O isotope，it is considered that the ore-forming fluids mainly come from the magma，and are mixed with small amounts of atmospheric precipitation.In Yuanzhuding deposit，the occurrence and its ore-forming mechanism of molybdenite are different from that of chalcopyrite.Molybdenite is mainly from the porphyry ore-body，and the atmospheric precipitation as well as the K-metasomation is the main reason for molybdenite precipitation.While chalcopyrite should be the product under the metasomatism of ore-forming fluids with surrounding rock for a long period of time.

Key words: Yuanzhuding, Porphyry rocks, Fluid inclusion, H-O isotope