Shallow water deltas are widely developed in lake environments. Under the modification of waves, offshore beachbar deposits can be formed at the delta front. The shallow water delta-offshore beach-bar system in the lacustrine basin can be an important oil and gas reservoir type, and its architectural characteristics are still unclear. In this paper, the sand bodies of the 2nd submember of the 2nd member of the Eocene Shahejie Formation (Es₂) in the 2nd Block of Shengtuo Oilfield, Dongying Sag, Bohai Bay Basin were finely dissected by using core and abundant well data. Combined with the results of sedimentary numerical simulation based on Delft3D, the architectural characteristics and formative mechanism of the shallow water delta-offshore beach-bar system in the lacustrine basin were explained. The study believes that the shallow water delta-offshore beach-bar system has obvious facies differentiation characteristics along the provenance direction, showing the evolution law of shallow water delta front to small-scale beach sand to large-scale bar sand to small-scale beach-bar sand. The shallow water delta front and large-scale offshore bar sand were formed simultaneously, while the small-scale beach sands developed later in areas between them. The front of the shallow delta shows the characteristics of wave influence. The number of distributary channel branches is small and the distributary channel is deeply cut in the middle of the mouth bar. The mouth bar is in the shape of a fan. After the distributary channel has extended a certain distance, it avulses and forms a new mouth bar laterally. Large-scale lenticular thick bar sands are formed proximal to the surf zone, initially developing on both sides of delta-front shoreline sands through the growth and coalescence of two small bar units. These composite features display biconvex thickness maxima (dual-thick centers) with a characteristic humpbacked morphology. The scale of the mouth bar and the large-scale bar sand is relatively large and the aspect ratio is high. The length-to-width ratio of the former is low, while the length-to-width ratio of the latter is large; the scale of the proximal and distal beach sand is small, and the aspect ratio is also small. This study provides reference for the fine-scale potential tapping of remaining oil in the Shengtuo Oilfield of the Dongying Sag.