This study systematically investigates the progressive failure and damage mechanisms of bedrock–overburden slopes under sequential rainfall–seismic action through large-scale shaking table tests (geometric similarity ratio 1:50) from a novel force–deformation coupling perspective. A new damage diagnosis method synergizing the displacement residual ratio and strain-field mapping is proposed. Key quantitative findings and innovations include as follows: (1) Rainfall infiltration raises saturation near the bedrock–overburden interface to ∼0.9 (from an initial moisture content of 6 %) and reduces peak soil strength by ∼50 %, creating a low-strength weak zone; (2) failure follows a progressive mechanism of “moisture weakening → dynamic driving → damage coalescence,” characterized by retrogressive sliding at the crest and multi-stage flow failure at the toe; (3) the acceleration amplification factor exhibits significant elevation and surface-proximity effects, with an abrupt increase in its growth rate at peak ground acceleration ≥0.4 g, serving as a dynamic indicator of the nonlinear damage stage; (4) methodological innovation: the displacement residual ratio is introduced. At 0.4 g, 0.4 at the crest versus 0.15 at the toe; at 0.5 g, these values increase to 0.75 and 0.68, respectively; 1.0 at 0.6 g. This index captures the elastic–plastic transition more sensitively than the cumulative displacement. Combined with spatiotemporal strain-field evolution, the damage propagation path from surface accumulation to crest concentration and toe interconnection is visually revealed, enabling the early identification of the potential slip surface; (5) scientific significance: antecedent rainfall is confirmed as a critical predisposing factor governing seismic failure modes. The proposed synergistic framework of displacement residual ratio and strain-field mapping provides a multi-dimensional quantitative basis for seismic design, monitoring, early warning, and risk management of slopes in high-intensity seismic regions.