The Yin"e Basin, located in the central-western part of Inner Mongolia Autonomous Region, consists of a series of Early Cretaceous rift depressions and forms part of the Late Mesozoic wide rift system in Northeast Asia. The Chagan Sag at the eastern margin of the basin is one of the few proven hydrocarbon-rich sags, whose Lower Cretaceous geological structure records the evolutionary process of multi-phase rifting. Conducting research on the geological structure and evolution of Chagan Sag is not only significant for revealing the evolution of Early Cretaceous intracontinental rift basins in northern China, but also provides valuable references for identifying hydrocarbon-rich sags and evaluating exploration targets in the region. Based on integrated datasets including 3D seismic surveys, drilling data, core samples, and field geological investigations, this study systematically investigates fault systems through structural analysis methods to characterize the grouping and evolutionary stages of major faults. The research identifies and delineates volcanic edifices to examine episodes of volcanic activity, reconstructs the Structural evolution since the Early Cretaceous using balanced cross-section techniques, and comprehensively analyzes the Early Cretaceous tectonic-sedimentary filling processes in the Chagan Depression. This study shows that the Chagan Sag exhibits well-developed volcanic-sedimentary interbeds, which can be vertically subdivided into three distinct structural layers: the rift structural layer, rift- depression transition structural layer, and depression structural layer. Within the rift sequence, three fault systems with NNW-, NNE-, and NE-trending orientations are identified, exhibiting an east-west zonation structural pattern. Three distinct episodes of volcanic activity are recognized in the Lower Cretaceous Suhongtu Formation, displaying a clear SE-to-NW migration trend throughout the sag. The results demonstrate that the Chagan Sag developed as a volcanically rift basin during the Early Cretaceous, having undergone a complete evolutionary sequence from rifting through rift-depression transition to depression since the Cretaceous. The rifting stage can be further subdivided into: (1) initial rifting phase, (2) intense rifting episode I, and (3) intense rifting episode II. Spatiotemporal variations in the activity of master faults were found to exert fundamental control on both the structural evolution of the sag and the migration patterns of volcanic activity.