The Western Hills and Ming Tombs (Shisanling) area in Beijing is characterized by extensively distributed Mesozoic volcanic rocks of the Tiaojishan Formation. In recent years, numbers of isotopic ages of volcanic rocks in Tiaojishan Formation have been obtained, however, they have a significant difference. The Western Hills can be divided into three sections, Ming Tombs area can be divided into four sections, systematic sampling were carried out in a typical cross section of the Tiaojishan Formation and were measured by LA ICP MS and SHRIMP zircon U Pb dating, respectively.Methods:The SHRIMP and LA ICP MS U Pb dating and whole rock geochemical analysis are employed to reveal the formation age and temporal as well as spatial characteristics of the Tiaojishan Formation.Results:By the geochemical analysis, the volcanic rocks in the Tiaojishan Formation in Ming Tombs area has a high Al2O3, CaO, Na2O, low TiO2 and MgO, fall into trachyandesite and dacite field in TAS diagram with enrichment of LREE and LILE ( K,Sr,Ba) and depletion of HREE and HFSE ( Nb,Ta,Ti), moreover, these intermediate—acidic rock has a high Sr/Y ratio, while the levels of Yb and Y is low. Combined with previous research results, the geochemical characteristics of the volcanic in Ming Tombs area is similar to the volcanic rocks in Westen Hills. By systematical sampling, the writers obtained the following isotopic ages:the age range of the Tiaojishan Formation in Western Hills is 152~ 137 Ma, and the age range of the Tiaojishan Formation in Ming Tombs area is 157~153Ma. Combined with the dating results of previous researchers, the volcano startup time in the Ming Tombs area is earlier than that in the Western Hills. Conclusions:the diachronism of unconformity gives rise to the differences in the age of Tiaojishan Formation between Ming Tombs and Western Hills. Meanwhile, the unconformity under the Tiaojishan Formation is a primary unconformity in Beijing.