Abstract: The Qagan Obao molybdenum deposit was discovered in the northern part of the Langshan Mountain. It is a porphyry muscovite quartz vein type deposit, and the ore-forming rock bodies are mainly K-feldspar granite, fine-grained granite and granite porphyry, with the zircon U-Pb mean age of （53.5 ± 3.3） Ma. Molybdenite 187Re-187Os isochron age of the Qagan Obao molybdenum deposit is （239.2 ± 5.8） Ma, with the rock-forming and ore-forming time difference being about 13 Ma, suggesting that the formation of the Qagan Obo molybdenum deposit experienced a long rock-forming and ore-forming evolution history. The Qagan Obo molybdenum deposit has the characteristics of the porphyry deposit, the ore deposit occurs in the inner contact zone with the country rock, but the mineralization took plade in muscovite quartz veins, mainly related to hydrothermal mineral muscovite. The deposit is therefore a special kind of porphyry molybdenum deposit.