The relative lake level is fundamental for constructing the sequence stratigraphy. However,considering the difficulty of ensuring the location of the lake level and the complexity of lacustrine rift basins,using traditional methods to establish the sequence stratigraphy is quite difficult. Thus,we introduced‘trajectory analysis’ in this study. We used well logging,lithological association,and seismic data to establish sequence stratigraphy of the lower part of the Second Member of Dongying Formation(corresponding to SQd₂)in the western part of Bozhong sag. Prograding sigmoidal clinoforms,prograding oblique clinoforms,small-scale mouth-shaped reflection,and parallel to sub-parallel reflection developed in SQd₂ of Bozhong sag. We interpreted four types of seismic reflection into three sedimentary systems,including river deltas,sub-lacustrine fans,and lakes. According to the direction of migration and the shape of shorelines,there are four types of shoreline trajectories identified,which are called ascending regression with convex-upward curvature(type-1),ascending regression with concave-upward curvature(type-2),descending regression with convex-upward curvature(type-3)and ascending transgression with concave-upward curvature(type-4),respectively. These four types of shoreline trajectories were related to the lowstand systems tract(LST),highstand systems tract(HST),falling stage systems tract(FSST),and transgressive systems tract(TST). Large-scale deltas developed during the late stage of SQd₂. The ascending regression with concave-upward curvature reflects the normal regression and large-scale deltas with‘thick topsets and thick foresets’ developed during this stage. Then,small-scale sub-lacustrine fans may develop due to gravity. The descending regression with convex-upward curvature is related to forced regression. During the stage,large-scale deltas with thin topsets and thick forestes developed. Thus,more sediment was transported into the deep lake,and large-scale sub-lacustrine fans were developed. In conclusion,we introduced‘trajectory analysis’ to divide an entire third-order sequence into four systems tracts. Then,we highlighted the importance of forced regression and the large-scale,rich sand sediment developed during this stage. Additionally,sub-lacustrine fans with relatively high sand ratios are always associated with rich-sandy deltas. The deltas and sub-lacustrine fans during the FSST are significant for reservoirs and lithologic traps.