Large-scale shaking table tests are conducted to investigate the seismic displacement relaxation zone in the reinforced slope by composite retaining structures. A new method was developed to determine the boundary line of displacement relaxation zone. The testing points corresponding to the selected allowable displacement value in displacement curves with different elevations are connected, and then a boundary line of the displacement relaxation zone is formed. It is shown that the range of the displacement relaxation zone increases with the increase of the amplitude of input seismic wave. The response of the slope displacement is gradually weakened from outside to inside of the slope under the action of composite retaining structures. The time corresponding to the peak displacement value of the slope is posterior to the time corresponding to the peak value of the input seismic wave. The slope between the relative height of 0.368 and the slope crest shows displacement coordination, and the time corresponding to the peak displacement value is close in this part but is shorter than the slope toe. The results presented in this study can be used to guide the understanding of the seismic displacement relaxation zone and the optimizing of the composite retaining structure.