The one-dimensional shear beam model method is unsuitable for determining the stress–strain relationship in the earth-rockfill dam foundation because of the constrained ground surface and the effects of the lateral normal stress. A centrifugal shaking table test was conducted on an earth-rockfill dam with overburden saturated sand foundation to analyze the soil acceleration response in the non- dam-covered area of the foundation, middle and toe of the dam under different input seismic loads. This test also obtained the variation characteristics of the horizontal dynamic earth pressure in the upper and middle layers of the dam foundation. An inverse calculation method was proposed to obtain shear stress–strain in the dam foundation soil, considering the influence of horizontal dynamic earth pressure. Comparing it with the stress–strain hysteretic loop inversed through the ideal one-dimensional shear beam model, the effect was analyzed about dynamic soil pressure on the dynamic shear modulus of the dam foundation soil. The test results show that the amplification effect of acceleration increases linearly from the bottom of the dam foundation to the top dam, and the variation of horizontal earth pressure cannot be ignored in the inverse analysis of shear stress–strain. Besides, the soil shear modulus decreases after considering the effect of horizontal dynamic earth pressure. As the depth increases, the effect of dynamic earth pressure on the shear modulus in the middle of the dam foundation is greater than that of the upper sand layer. Moreover, the relative calculation error increases with the increase of depth and peak acceleration of the input seismic wave.