For a large area of shallow slope covered with weathered soils

when the groundwater table or the impermeable bedrock is shallow

intense rainfall will result in the air in soils being closed. With the wetting front moving downward

air pressure increases constantly. Closed air pressure not only reduces rainwater infiltration rate of the soil in slope

but also has a significant effect on slope stability. This paper analyzed the formation of air pressure and its related theories

and suggested that a pressure head was used to study slope stability(Hc is a value of water head related to the pore size distribution; hd is an air-closing water head). Combining the Mohr-Coulomb failure criterion of unsaturated soil and the limit-equilibrium method

the closed air pressure head is applied to slope stability analysis; and the stability analysis model is established considering air pressure. The comparison was also carried out with the classical stability analysis method; and a concept of effect rate of air pressure is also defined. It is found that closed air pressure significantly reduce the safety factor of slope stability; the classical stability analysis method of infinite slope is more dangerous; research results in this study of infinite slope safety prediction have a great guide significance.