Abstract: Seismic activity induces strong dynamic disturbances on steep, precarious rock masses, promoting the propagation and coalescence of cracks at the tips of the main control fractures and ultimately triggering collapse. The collapse of precarious rock masses is a common type of geological hazard in the mountainous regions of Guangxi. Addressing the limitations of existing research in understanding the dynamic response characteristics and mechanisms of steep, precarious rock masses under seismic loading, this study investigates a typical overturned precarious rock mass in Nongjian Village, Dahua County, Guangxi. Employing the PFC2D numerical simulation method to investigate the mechanical mechanisms of crack initiation and propagation from a microscopic perspective. Furthermore, it analyzes the effects of the angle between second level and main control fractures, as well as different seismic loading modes, on the stability of the rock mass. The results show that (1) The collapse of the toppling-type precarious rock mass is essentially a tensile failure, with cracks initiating and propagating rapidly at the peak tensile displacement. (2) As the angle between the second level fracture and the main control fracture increases, the failure mode of the precarious rock mass transitions from sliding to toppling. The presence of second level fracture significantly alters the stress distribution and energy release characteristics at the tips of the main control fracture, thus affecting overall stability. (3) The loading method of seismic wave significantly influences the failure of precarious rock masses. Horizontal seismic wave has a significantly stronger failure-inducing effect than vertical ones, while the superposition of multi-directional seismic waves substantially amplifies the energy released during collapse, leading to more severe failure. The findings are of significant practical importance for the stability assessment and the design of mitigation measures for precarious rock masses under seismic conditions.