Abstract: The Xingfuzhilu Formation, previously defined as Early Triassic strata, is distributed in southern Da Hinggan Mountains. The Lower Triassic strata are of fluvial-lacus-trine facies with red-bed features. The stratotype of the third member of the formation was reinvestigated by the authors. The age of the stratotype section is early Early Triassic, as evidenced by the newly-discovered conchostracans, pollen and spores. The volcanic tuffs of the lower part of the section were discovered for the first time. LA-ICP-MS dating of two tuff samples yielded ages of 252.6±1.7Ma(MSWD=0.64) and 249.9±1.6Ma(MSWD=0.22). In combination with the biological characteristics, geographical division and timing of the magmatism, it is concluded that the Xingfuzhilu Formation characterized by fluvial-lacus-trine facies and red beds was formed by the collisional collage of the North China Platform with the Siberia Plate in Early Triassic.