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The Huolinhe Formation in the Huolinhe Basin, eastern Inner Mongolia is one of the most important Lower Cretaceous coal-bearing strata in China, yielding abundant, diverse, and well-preserved plant fossils. Its precise age, however, is poorly known due to lack of associated marine deposits and volcanic beds. Here we present U-Pb zircon ages, and the associated palynological assemblages of an ash layer of the Huolinhe Formation recently discovered at the Zhahanaoer open-cast coal mine in Jarud Banner. Stratigraphic analyses based on boreholes suggest that the ash layer occurs near the bottom of the “lower coal-bearing member” of the Huolinhe Formation. U-Pb zircon geochronology using the SIMS method constrains the depositional age of the ash layer to be 125.6 1.0 Ma (late Barremian–earliest Aptian), and this is consistent with the result from LA-ICP-MS analyses of the same sample. A late Barremian–earliest Aptian age for the ash layer is also supported by the palynological assemblage associated with the layer, in which the pollen of gymnosperms and the spores of ferns and bryophytes are dominant, angiosperm pollen is very rare and represented by only Clavatipollenites. This study contributes important new data for understanding the age of the entire Huolinhe Formation and also provides a more precise maximum age for the key plant fossils preserved in the deposits above the ash layer.