Dynamic response of saturated porous medium is of great significance in many engineering fields. The consideration of varying porosity helps to reasonably reveal the related mechanical behavior of saturated porous medium such as soil. Dynamic porosity model is combined with u-U-p equation representing the dynamic characteristic of saturated porous medium. And a new nonlinear dynamic model is established. Comsol Multiphysics PDE is used to obtain its numerical solution. On this basis

the change of porosity

deformation and pore water pressure of two-dimension saturated soil are developed under two different kinds of permeable conditions and excited by harmonic load on the surface. The result shows that the porosity relates directly to the volumetric strain and pore water pressure of soil. The loading process decreases soil porosity

and enhances the interaction between solid skeleton and pore fluid increases

therefore increases the resistance applied on solid skeleton. The dimensionless vertical displacement and pore water pressure are also low under constant porosity condition. It is useful to increase the research rationality of mechanical behavior of saturated porous medium such as soil if the dynamic porosity is considered. On the other hand

pore fluid can discharge freely from permeable top surface. So

the load that is born by soil skeleton is greater

the porosity

deformation and pore water pressure are also greater comparing with the condition under impermeable top surface .