The optimum foam system was composed of 0.5% foaming agent ZY (petroleum sulfonate) and air in gas liquid volume ratio1.2:1. The foam’s volume was 400 mL, foam half-life was 540 s and apparent viscosity of foam was 1210 mPa·s. The mathematical expression of effective viscosity μ_X of air foam penetrating in pour medium at different sites on compacted sand stones spacing normalized from injection was deduced based on the Darcy and quantity balance laws. And the shear rate of γ foam penetrating was deduced according to the core pore throat diameter and the foam seepage velocity. Moreover, it was calculated that the relationship between μ_X and γ when the foam system was flooding in porous media. And the mathematical expression was μ_X=Kγ^n-1 . The consistency factor was 375.94 and the power law index was 0.33.