The direction of inertial force under seismic conditions has a significant impact on the equilibrium state of combined structures, soil, and rock masses. To gain a deep understanding of the interaction characteristics between the pile-plate wall combined support structure and the bedrock and overburden layer slope, a series of large-scale shaking table model tests on the slope reinforced by combined structures under sinusoidal wave load were conducted, using a high steep slope at the tunnel portal in the southwestern region as a prototype. The analysis of the time-history characteristics of the pile-soil and the geogrid-anchor cable-slope under the direction of inertial force was considered. The results indicate: (1) Under the action of inertial force, the lattice anchor cable-soil interaction and pile-soil interaction show an approximate time-history consistency law, but the phase difference between the peak value of the inertial force of the slope reinforcement structure along the elevation cannot be ignored. (2) The interaction between the structure and the slope soil depends on the relative inertial motion, and the pile-bedrock interaction is determined by the deformation state of the cantilever section. (3) The inertial force reaches the peak value of the free surface, the dynamic earth pressure reaches the peak value, the pile-soil displacement difference reaches the positive peak value, the pile body shows an active failure state of camber, and the stress on the front side of the anchoring section increases due to camber, so the design using Coulomb active earth pressure should be revised based on the pile-soil conditions. (4) Under sinusoidal wave conditions, the inertial force and deformation of the slope are crucial factors affecting the force distribution characteristics of the combined retaining structure along the elevation. The design of the combined structure needs to focus on the stress concentration area at the foot of the slope and the inertial force amplification area at the top of the slope. This research can provide theoretical support for the seismic reinforcement design of combined structures.