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 Fracture 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, based on the generalised fault theory, and through the structural analysis of the 3D seismic data system, they revealed that the tectonic 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, but they are also strongly similar to the features of the structural deformation under oblique extension. Furthermore, our article proposes a new insight into the mechanisms of the No. 1 fault based on the analysis of the structural features of the No. 1 fault and also combined with the analogue modelling. The results show that the No.1 fault has obvious characteristics of an oblique-slip normal fault, and there are significant differences in the amount of strike-slip and dip-slip displacements in different areas along the fault strike, as well as the control of sedimentation in 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 r 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.