The key to simulating the mechanical properties of earth-rockfill dam materials is selecting a reasonable constitutive model. The traditional constitutive model targets specific earth-rockfill dam materials and involves many hypotheses in theory, resulting in large common errors in applications. The Pastor-Zienkiewicz model (P-Z model) based on generalized plastic mechanics theory has a strict theoretical base, but it involves many parameters. Moreover, the determination of parameter a entails high discreteness because of experimental conditions, and this discreteness influences the accuracy of simulation analysis. The mechanical properties of earth-rockfill dam materials were simulated using the P-Z model in this study to optimize parameter a of the model and decrease its discreteness. First, conventional triaxial shear test results of earth-rock materials were denoised with the empirical mode decomposition method. Second, a method to optimize parameter a was proposed by using cloud theory. Lastly, a numerical simulation analysis of the triaxial shearing test was performed with the optimized P-Z model. Research results demonstrate that the entropy error and hyper entropy error rates of the triaxial shearing test data decline by 35% and 67%, respectively, and the cloud droplet range of parameter a is narrowed by 32%. The discreteness of the values decreases significantly. The simulation calculation results are consistent with the test results, thereby confirming the reasonability of the proposed parameter optimization method. The optimized P-Z model can simulate the mechanical properties of earth-rockfill dam materials accurately. This study can provide a theoretical reference for finite element simulation analysis of earth-rockfill dam structures.