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In the framework of generalized plasticity theory, this paper presents a new elastoplastic model to characterize complicated softening/hardening and dilation/contraction behaviors of sand–gravel composites in triaxial tests. The model has six parameters that are determined by the conventional triaxial test directly, which is of great practical interest to engineers. The dilatancy equation that is able to describe the dilatancy of sand–gravel composites during the whole shearing process is incorporated into the model. The advantage of the proposed model in predicting the dilatancy behavior of sand–gravel composites is demonstrated by comparing it with three widely used dilatancy equations. A set of drained triaxial compression tests were launched to examine the performance of the proposed model. In addition, the applicability of the model is also confirmed by sand–gravel composites tests covering a wider range of confining pressure in previous literature. The generality of the model on other granular materials including rockfill, Ottawa sand, calcareous sand, cement-sand–gravel material, and glass beads mixtures is also verified by comparing the experimental results with the corresponding fitting results. Furthermore, the proposed model is programmed into the nonlinear finite element program GEODYNA and applied to the numerical simulation of high concrete-faced sand–gravel dams. Summarizing the fitting and numerical results comprehensively, the constitutive model proposed in this study is capable of characterizing the mechanical behaviors of granular materials and can provide a powerful tool for geotechnical engineering. [ABSTRACT FROM AUTHOR]