Abstract: The gas phrase permeability of core sample is one of the important indexes for laboratory evaluation of formation damage, and the permeability changes before and after the contamination by drilling fluid and other medium must be measured and calculated accurately. Keeping the water saturation of core sample at a relatively stable value is very important for accurate measurement and calculation. Thus a laboratory testing method, namely wetted gas steady displacement is presented for maintaining the irreducible water saturation of core sample in a relatively stable level.