Abstract: Abstract: The Dashimengou molybdenum deposit in Songxian County of Henan Province is a newly discovered large molybdenum deposit in western Henan. Five samples of molybdenite-bearing breccia and there samples of molybdenite-bearing broken quartz related to mineralization were selected for precise molybdenite Re-Os dating, and the model ages obtained are from (156.1±3.4) Ma to (228.0±4.1) Ma. The model ages can be obviously divided into two groups: the Re-Os model ages of the first group range from (208.4±3.1) Ma to (228.0±4.1) Ma, with an average model age of (218.07±4.3) Ma and a weighted average model age of (217.1±8.5) Ma; the second group range from (156.1±3.4) Ma to (160.9±3.3) Ma, with an average model age of (158.6±3.35) Ma and a weighted average model age of (158.6±4.7) Ma. The results show that the Dashimengou molybdenum deposit was formed in the Indosinian and Late Yanshanian periods. The ore-forming processes of the two phases were related to two cryptoexplosion events of the hydrothermal breccia. The rock-forming and ore-forming activity of Indosinian period occurred in the transition period of the tectonic regime from compression to extension after collision orogenic movement of the East Qinling molybdenum ore belt. The upwelling of partially melted upper mantle and lower crust produced cryptoexplosion at the intersections of NW-and NE-trending structures. Late Yanshanian mineralization seems to have been the inheritance, transformation and superposition of the Indosinian mineralization, and the ore-forming processes of these two periods constituted the full cycle of the Mesozoic metallogenic explosion of the East Qinling molybdenum ore belt.