The triaxial instrument has a scale effect due to the particle size limitation. The Xinjiang Altash dam building materials were taken as the research object, and through the large-scale triaxial numerical simulation test, the influences of scale effect on the mechanical properties and nonlinear constitutive model of gravel materials were studied from the macro and micro perspectives. Results showed that, with the increase of confining pressure, the extreme value of deviatoric stress of the sample increased, the degree of shear shrinkage was significant and the dilatancy characteristics were suppressed. The peak strength was increased with increasing specimen size, and the specimen transitioned from shear shrinkage to shear expansion. With the increase of specimen size, the internal friction angle component, the elastic modulus parameter, and the bulk modulus parameter of Duncan's model parameters were significantly increased, and the effects of other parameters were negligible. The relationship between internal friction angle component and particle size for gravel was expressed as a logarithmic function, and the relationships between elastic modulus parameter, bulk modulus parameter and specimen size were expressed as linear functions. As the size of the specimen increased, the range of the deformation concentration zone was widely expanded, and its form gradually changed from the "X" type to the "X+x" or "X+x+x" type deformation concentration zone. As the specimen size increased, the initial coordination number was increased. During loading, the coordination number increased and the specimen was dominated by shear shrinkage. As the specimen size increased, the coordination number was shown to decrease gradually, and the shear shrinkage was suppressed.