Abstract: Great Sanjiang region is an important oil and gas resources succeeding district in eastern China. During the Early Cretaceous period, this region was once a unified lake basin, and then broken up. The problem as to the determination of the basin boundary and the reconstruction of the basin prototype is one of the key geological problems in this region. Previous study on the basin boundary was mainly focused on theoretical speculation and seemed lack of actual evidence. In this study, a set of conglomerates was found at the bottom of Muling Formation in Linkou basin. Based on an analysis of gravel composition and sorting as well as sedimentary characteristics, combined with the paleocurrent recovery and seismic data interpretation, the authors have concluded that the set of conglomerate is on the whole of alluvial fan facies, the river conglomerate deposition and overflow conglomerate deposition constitute the dominant depositions, and the provenance mainly came from the Zhangguangcai Mountain in the west of the basin. Combined with the seismic data analysis, it is held that the deposition model of the study area was the direct transition from the alluvial fan to the shallow lake subfacies in the period of Muling Formation. This study not only provides direct evidence for the determination of the western border of the Great Sanjiang Basin, but also points out the direction for the future oil and gas exploration in this region.