Abstract: Sand gravel soil is a commonly used building material in earth rock dam engineering, and its maximum dynamic elastic modulus and maximum dynamic shear modulus are important parameters for seismic design of earth rock dams. However, the existence of scaling effect leads to errors between indoor test results and true values of maximum dynamic modulus, which seriously affects the safety design of earth rock dams. There is currently limited research on the impact of scaling effect on the maximum dynamic modulus, and there is a lack of accurate calculation methods. Therefore, it is necessary to explore the variation law of maximum dynamic characteristics of sand and gravel soil under the influence of scaling effect and propose calculation methods. According to previous studies, the variation in the maximum particle size d_max and the gradation structure (represented by grading area S) are the primary causes of scale effects observed in reduced-scale specimens. By changing d_max or S, 18 groups of specimens with different gradations of coarse-grained soils were designed. A series of large-scale dynamic triaxial tests for each specimen were conducted by a large-scale dynamic triaxial apparatus to quantitatively study the influence of scale effects on the maximum dynamic elastic modulus E_dmax and the maximum dynamic shear modulus G_dmax of Sand gravel soil. The test results show that when d_max remains constant, as S increases, E_dmax and G_dmax of the sample first increase and then decrease. An empirical formula that can describe the relationship between E_dmax, G_dmax and S was established; When S remains constant, as d_max increases, the sample E_dmax and G_dmax gradually increase, and E_dmax, G_dmax and d_max exhibit a logarithmic function relationship; By coupling the relationship between S and d_max and their effects on E_dmax and G_dmax, a prediction model for E_dmax and G_dmax of sand and gravel materials considering scaling effect was established, and its accuracy was verified.