A fractal model for the rockfill particle-size distribution is employed and the range of fractal dimension is estimated through fitting the gradation curves of 32 rockfill materials from different rockfill dams. The gradation of rock materials possesses good fractal behavior, so that the fractal dimension D can be used as a new index to describe the gradation characteristics of rockfill materials. Based on the analysis of statistics, it is shown that fractal dimension D varies from 2.348 to 2.699, and it is about 2.6 for most dam materials. Six sets of gradation curves for cushion layer of Gushui rockfill dam are designed based on the fractal model. By employing the stochastic granular discontinuous deformation method (SGDD), the effects of fractal dimension on compaction capacity and the macroscopic and mesoscopic mechanical properties of rockfill materials are analyzed. As fractal dimension varies from 2 to 2.8, the void ratio decreases first and then increases, the material achieves its maximum compaction at D = 2.7. The degree of heterogeneity of force chains increases with the increasing of fractal dimension, and attains the maximum at D = 2.8. By considering both the compaction capacity and the force chain heterogeneity, it is found that the gradation at D = 2.7 is the optimum gradation.