After air entrance, an unsaturated area is formed in saturated soil. So, in fact, air permeation in saturated soil aquifer is an unsaturated permeation problem. Based on the constitutive equation, which is deduced from porous media theory with consideration of the compressibility of air, the characteristics of air permeation in saturated soil aquifer in Shanghai area is studied through laboratory tests. Results show that the air permeation in saturated clayey soil has a threshold of pressure about 100~200 kPa, which nearly equals to the air-entry value of the corresponding soil. The air permeation velocity and pressure gradient curve in the soil show clearly in step-phase. The main reason for this phenomenon may be due to that in saturated porous soil, all the channels are full of water. At the initial stage of air entrance, only a small mount of absorbed water can move under low air pressure. And with the air pressure increases, the air permeation channel expands, more air can be transmitted. The study of air permeation in saturated soft soil is helpful for air pneumatic underground engineering construction activities in saturated soft soils.