Abstract: By means of field geological investigation, geological section survey, drilling core logging, fossil study and isotopic dating, combined with the comprehensive analysis of previous data, it is concluded that the Xinmin Formation is mainly a set of volcanic rocks and pyroclastic rocks with coal deposits, bearing mid flora of Coniopteris-Phoenicopsis, bivalve fossil assemblage of Ferganoconcha tomiensis-F. elongate and sporopollen assemblage of Osmundacidites-Lycopodiumsporites-Piceites, which belong to Middle Jurassic. The zircon U-Pb age of volcanic rocks is 161.6 Ma; while that of detrital rocks is between 175±2 Ma and 2537±20 Ma. The youngest age of detrital rocks (175±2 Ma) represents the lower deposition limit of Xinmin Formation. The research results would provide essential data and scientific basis for the Middle Jurassic stratigraphic classification and correlation, as well as the establishment and perfection of the Mesozoic stratigraphic framework in Southeastern Daxinganling Mountains.