Through the laboratory experiments, the mechanical properties of sandy gravel are obtained. Then, based on the slump tests, a modified sandy gravel model is developed by PFC3D (particle flow code of three dimensions), to which relevant microscopic parameters are assigned to numerically simulate the laboratory experiments. The laboratory test results are compared with that of the numerical simulation by PFC3D, which shows their perfect agreement. Thus the particle flow method is able to better simulate the process of the laboratory experiments of sandy gravel. A new approach for the study of the physical and mechanical properties of modified sandy gravel is proposed in the research to overcome the limitations of conventional tests.