Abstract: The structure, formation and mechanisms of complex fault systems developed in extensional basins remain a challenging issue in the analysis of hydrocarbon-bearing basins.We used sandbox analogy modeling method to investigate the development and formation mechanism of rollover and crestal collapse faults above typical extensional listric faults in the Huimin depression of the Bohai Bay basin.By designing two sets of controlled experiments(E1 and E2), the influence and control mechanisms of the development of detachment and internal depressional faults in the Huimin Sag were investigated.Analogy experiments E1 reproduce the formation process of rollover and crestal collapse faults in the Huimin Sag, indicating that the Ningnan fault actually controls the morphology of the hanging wall and the development of the secondary fault system.The crestal collapse fault system is asymmetric and is composed of antithetic- and synthetic faults, the newly developed conjugate faults are contemporaneously active, and the fault activity shows a pattern of lateral migration.The statistics of fault displacement indicate that two sets of rupture surfaces are simultaneously active and in conjugate relationship.This is consistent with the Coulomb-Mohr failure criterion within the extensional wedge, indicating that the wedge is at or near the critical state.In another set of E2 experiment, the development of the Linyi and Xiakou faults offset the crestal collapse faults formed earlier, and the Linnan depression began to form and widen, eventually forming in a structural pattern consistent with the present-day structural profile.This study reveals the development mechanism of rollover and its associated crestal collapse fault in typical extensional fault-related folds, which has important implications for extensional fault system analysis.