The Xingjiashan Mo-W deposit is the only large- size molybdenum deposit in Jiaodong, which is well known for numerous gold orefields. In order to find out the ore- forming mechanism of this deposit, the authors conducted systematic geological survey and geochemical analysis in the Xingjiashan Mo deposit. The results show that the Xingjiashan Mo deposit has experienced magmatic hydrothermal period (comprising skarn stage, quartz- sulfides stage and carbonates stage) and supergene oxidation period. Molybdenite mainly occurs in garnet-vesuvianite skarn of the quartz- sulfidies stage. Geochemical data indicate that substances like SiO₂, Al₂O₃, TiO₂ and Fe₂O₃ migrated into the skarn from Xingfushan monozie granite, while CaO and MgO were brought outward. The migration of oxides provided enough substance for the formation of skarn minerals. The obvious similarity of REE patterns and spider diagrams between skarn rocks, stock and marble implies a closely genetic link between them. Besides, the values of δEu and δCe rising from ores to remote Pb-Zn mineralization marble might record the environment change from reducing to oxidizing in the post- ore stage. The C-O isotopic compositions of calcite veins also show the granitic origin features. In addition, the low Re content in molybdenum of this deposit may indicate that Mo was derived from the lower crust. The Xingjiashan Mo deposit is a typical contact metasomatic skarn deposit formed in early Yanshanian period.