Abstract: The Xiaohulishan molybdenum deposit is a medium-size concealed porphyry molybdenum deposit recently discovered in Beishan area, Inner Mongolia. The authors studied isotopes of the ore-bearing porphyries and the quartz vein type molybdenum deposit in late mineralization stage, and obtained the zircon LA-ICP-MS U-Pb age of 216.9Ma±0.5Ma（MSWD=0.41）for Xiaohulishan porphyry granite, which is close to the Re-Os model age of 213.2Ma±4.6Ma (MSWD=1.3) for molybdenite, thus providing isotope age evidence for porphyry genesis of the Xiaohulishan molybdenum deposit. The molybdenite Re-Os isotope age in late metallogenic stage is 213.2Ma±4.6Ma(MSWD=1.3). The ore-forming porphyry of the Xiaohulishan Mo deposit was distributed along the Heihe River which was controlled by NEE-trending faults that cut the early NW-trending regional structure, implying that 216-220Ma was the age restriction for the evolution of the post-collision stage to the intraplate extension stage in Beishan area. Importance should be attached to mineralization in Indosinian period in Beishan area.