Abstract: The Balonggongga'er Formation is distributed in the South Qilian Mountain and belongs to marine terrestrial clastic rock series, and its geological epoch has aroused much attention. Based on 1:50000 regional geological survey, the authors systematically studied the lithological characteristics, basic sequence, sedimentary structure, lithofacies change, and age of the Balonggongga'er For-mation. Some basaltic andesite samples within the Balonggongga'er Formation were identified, and LA-ICP-MS zircon U-Pb analy-ses indicate that the basaltic andesite was formed at 427.2±2.8Ma, belonging to the Early Silurian period. In combination with previous achievements, the authors have further confirmed that the Balonggongga'er Formation was formed in Early Silurian period. These re-sults are of importance for the study of the Early Paleozoic stratigraphic sequence and tectonic evolution in the South Qilian Mountain.