The Yinggehai basin has long been of great interest to academics and the petroleum industry due to its special tectonic location (the area of the Red River fault zone into the sea) and its abundant natural gas resources. Since the Indo- Eurasian collision- extrusion model was proposed to explain the deformation in Southeast Asia, the Yinggehai basin has been regarded as a typical pull- apart basin. However, there is a distinct mismatch between these opinions and the structural features revealed by the 3- D (three- dimensional) seismic data. Our study is based on the “generalised fault theory”, using 3D seismic data to investigate the structural features of the No.1 fault systematically, and combining it with sandbox simulation experiments, we propose a new insight into the mechanism of the No.1 fault. The results show that the structural features and the distribution of fault distances of the boundary fault (No.1 fault) in the Yinggehai basin are not similar to those of the traditional strike- slip faults. Still, they are also strongly similar to the features of the fault deformation under oblique extension. These faults are oblique- slip normal faults with significant differences in the amount of strike- slip and dip- slip displacements at the various segments, as well as in the control of sedimentary filling of the basin. The boundary fault was an oblique- slip normal fault due to the reactivation of pre- existing structures under a north- south extensional stress regime during the Oligocene- Miocene period, and these insights were also verified by analogue modelling. A new insight into the mechanism of the No.1 fault, which theoretically reveals the mechanism of the Yinggehai basin and can provide guidance for the structural analysis of the Yinggehai basin and its later oil and gas exploration and development.