Abstract: The porosity is a macro-variable which can represent partly micro-structures of granular materials. Based on the continuum model of granular materials, an evolution formula for the porosity with local average volumetric strain of granular assembly is deduced. Provided the change of grain volume is uniform, and the formula is allied with pore water state equation, the relationship among the pore water pressures and the porosity, the volumetric module of grain and the volumetric strain of grain for saturated granular materials is presented. The results obtained can be applied to the numerical simulation of fluid-solid coupling for statured granular materials or to the multi-scale analysis of fluid-solid seepage for porous materials.