Abstract: The Jiguanshan Mo deposit is one of the important and large Mo deposits in the Xar Moron molybdenum metallogenic belt. Through studying the petrographic and geochemical characteristics of ore- bearing rocks in the Jiguanshan porphyry molybdenum deposit, the authors have found that the rocks are mainly granite, rhyolite, rhyolite porphyry, trachyte andesite, trachyte porphyry and diabase. They belong to high-K calc-alkaline series and shoshonitic series, and lie between the metaluminous field and the peraluminous field. Rare earth elements of samples show a distribution pattern of LREE enrichment, with LREE/HREE ratios being 3.94~7.87 and (La/Yb)N ratios 3.59~8.19. Except diabase, all the rocks show the loss of europium(δEu=0.10~0.68).Trace elements spider diagram of samples shows the enrichment of large ion lithophile elements and depletion of high field strength elements. According to the geochemical characteristics of ore-bearing rocks, the authors infer that the magmatic source region of intermediate and acidic rocks was the crust, there probably existed fractional crystallization in magma evolution, and the evolution process might have included the fractional crystallization of apatite and biotite, or the differentiation of hydrothermal solution of phosphorus with rich volatile matter. In combination with regional geological evolution and metallogenic characteristics of the porphyry molybdenum deposit, the authors hold that the deposit was formed in an extensional environment after the strong extrusion stage of Yanshanian movement, or resulted from the " creeping " and " stretching " process of East Asia continental crust.