Abstract: Along the direction of working face advancing, gob area can be divided into two sub-areas, named as freedom accumulation area and pressure accumualtion area. Based on the theories of overlying strata movement, a model is deduced to describe the porosity distribution in gob area. This model is not only continuously derivative inside the both two sub-areas above mentioned, but also it is continues on the boundary between the two sub-areas, and this characteristic can be expected beneficial to the stability of numerical solution of gas seepage in gob area.