Abstract: The Chaihe Formation of the Lower Jurassic is first defined in the northern Daxing' anling Range, which is a suite of sedimentary rocks and intermediate volcanic rocks. At the bottom of sedimentary rocks, we found plant molecular in the strata of Early Jurassic, namely Podozamites sp. and Equisetites sp. SHRIMIP zircon U-Pb age of the trachyandesites is 183.8±3.9Ma. The ε_Hf(t) values of the zircons of trachyandesites range from +9.7~+11.4. The T_DM2 model ages yield 495~612Ma.This fact indicated that the rocks could have originated by partial melting of juvenile crust, and the accretion age of the Eergu'Na block's main part was Neoproterozoic. The volcanic rocks of Chaihe Formation formed in the environment of active continental arc, relating to the subduction of Mongolia Okhotsk ocean to the Southeast direction in the Early Jurassic. The determination of the Chaihe Formation of the Lower Jurassic indicates the volcanic activity in the Lower Jurassic in the Daxing'anling Range. The establishment and introduction of Chaihe Formation is of great geological significance for the study of tectonic-magmatic activities in the Mesozoic, the comparison of Mesozoic volcanic strata and regional tectonic evolution.