To investigate the system nonlinear redundancy performance of a multi-girder steel plate composite girder bridge under asymmetric extreme loads, three bending failure modes of the system were identified along the force transmission path of the structure and the rebalancing process of the system. A new performance indicator was derived for quantitatively evaluating the elastic distribution and plastic redistribution of the system, and the redundancy evaluation method of time-varying system was established considering the time-varying variability of load mechanism. The management program of time-varying evolution parameter set of element fibers was written based on MATLAB/OpenSEES, and the influence of material deterioration on the rebalancing and internal force redistribution of the system was studied. The overall elastoplastic process analysis of the structure under asymmetric load was carried out based on the numerical increment algorithm. The accuracy of the numerical model was verified by comparing it with the failure tests at the component-system level, and then a nonlinear redundant numerical simulation method of time-varying system based on fiber macro-element was established. Results showed that material deterioration caused the transformation of the system from multi-girder cooperative bearing mechanism to single beam bearing mechanism, and resulted in varying degrees of degradation of member ductility, system redundancy, and reliability. Fewer beams or fewer transverse connections, while better suited for on-site rapid assembly, were prone to insufficient component safety and system redundancy in the service life. The failure of main beam, middle transverse joint, and end transverse joint had a great impact on the bearing performance of the system, which were key members of steel plate composite beam bridge, whereas the sensitivity of other transverse connections was low, which were non-key components.