Abstract: The Dajing deposit was previously regarded as a magmatic hydrothermal type deposit.The orebodies are controlled by the NW-and NWW-trending faults and occur in Permian Linxi Formation. The zircon samples collected during the research were dated by using LA-ICP-MS zircon U-Pb method, and the authors obtained the following results:the zircon U-Pb ages are 164.1±0.8Ma (MSWD=0.60, n=16) and 253.8 ±1.1Ma (MSWD=0.96, n=14) for felsite dikes and andesite porphyry dikes, respectively. Felsite dikes were formed in a collisional setting probably related to the closure of Mongolia-Okhotsk Ocean and post collision, while andesite porphyry dikes were formed in a tectonic setting possibly related to the collision between Siberian Plate and North China Plate. Based on the zircon U-Pb ages of dikes obtained, the authors have reached the conclusion that the ore-forming process of the Dajing polymetallic deposit occurred later than 164Ma. The most intense volcanic-magmatic activities of the Da Hinggan Mountains took place during Middle Yanshanian period. It is therefore held that the ore-forming process of the Dajing polymetallic deposit occurred approximately in Late Jurassic-Early Cretaceous.