The physical model and the computational system for powder packing process simulation are presented, and the high performance computing methods of simulating the random packing of mixed particles with different sizes are studied. In the simulation system the effects of gravity, contact forces, damping, friction, van der Waals force etc are take into account, therby forming several kinds of mechanical models and damping models. The system is very suitable for the three\|dimensional simulation of large scale powder packing process. Finally, to demonstrate the usefulness of the simulation system, two typical applications are presented in the paper. One is to simulate the random packing of binary mixture with size ratio 10, and when the number of small particles is 300 times as large as that of big particles, the maximal packing density （volume fraction） is achieved to be 0.824, and the other is to simulate the random mixed packing dynamics of particles with two different densities. The segregation phenomenon and clustering phenomenon take place obviously after the packing process has come to an end. The physical model and the simulation system presented in this paper are not only suitable for the study of powder packing process, but also applicable to the packing process simulation of spherical objects.