///The ore-forming age of Xiaohulishan molybdenum-polymetallic deposit in Beishan area, Inner Mongolia, has been studied in this paper based on the Re-Os isotopic age of molybdenite in ore and the ²⁰⁶Pb/²³⁸U age of zircon in the rock obtained recently. It is put forward that the genetic type of this ore deposit belongs to porphyry molybdenum deposit. The results show conclusions as the following: the ore-bearing rock was formed at (246.8 ± 1.7) Ma, which belonged to the Early Triassic; mineralization took place at the (220.8 ± 2.2) Ma, as in the mid-Triassic; the two ages reflect the time trajectory of the rock-formation→mineralization, which represents the time domain of mineralization process of porphyry molybdenum-polymetallic ore-forming system in the Xiaohulishan of Beishan area, Inner Mongolia.