Abstract: Using the PFC^3D as the research tool, accompanied with the laboratory tests, the sand-fall molding process and porosity variation of model are analyzed. Firstly, the falling order and arching phenomenon are studied. Thus, the flow model for the straight hopper with a central bottom hole is suggested, and an optimized square outlet is adopted in the laboratory tests. Subsequently, the PFC^3D is utilized to simulate and to verify the influences of height, aperture and velocity of outlet on the model porosity. The research results show that the particles leak densely when the aperture increases, thus, the particles of model can not adjust their positions in time, and the porosity remains large. With the increase of the velocity of outlet, the times of stacking and colliding both increase, and the porosity decreases. When the falling height increases, the kinetic energy is amplified in collision, and the porosity decreases. For the above mentioned factors, both the physical and the numerical tests show the same rules, and the suggested multivariate correlation model has a good adaptability.