Mesoproterozoic and Epiproterozoic carbonate strata are widely distributed in southern Zhangjiakou. Located in the Wumishan Formation in Chicheng County, the Liangjiagou Pb-Zn-Ag polymetallic deposit is a typical polymetallic deposit in the carbonate sedimentary cover of southern Zhangjiakou. Due to lack of data of ore-forming epoch of the deposit, there long exist two viewpoints concerning the genesis of the deposit, i.e., syngenetic sedimentation and epigenetic transformation. The ages of the Liangjiagou deposit were determined by the Rb- Sr isochron method for sphalerite and a group of mineral assemblage samples (pyrite and sphalerite), and the results are 126 Ma and 130.7 Ma respectively. The ages of the Liangjiagou deposit were also determined by the Sm-Nd isochron method for sphalerite and a group of mineral assemblage samples (sphalerite and pyrite), and the results are 130.5 Ma and 131 Ma respectively. The metallogenic epochs measured by the Rb-Sr and Sm-Nd isochron methods are very close to each other. Combined with regional geological background of Zhangjiakou, the authors hold that the mineralization resulted from hydrothermal activities in the Yanshanian tectonic movement period.