Abstract: The effects of pore-water pressure on the apex post characteristics of stress-strain curves for rock mass are analyzed from the macroscopic and microcosmic mechanisms. An apex post modulus Epost which indicates the effect of pore-water pressure on stress-strain curves transition is defined. Bycombining with the stiffness criterion of landslide, the effect of pore-water pressure on stress-strain curves apex post slope is analyzed. Along with the increase of pore-water pressure, the apex post curve slope becomes steep, and the apex post stiffness increases. The homogeneity and brittleness of slide surface medium increases and the stiffness ratio k decreases. When the system conditions don't change, corresponding with k=1, the pore-water pressure has a critical value pwcritical. When the pore-water pressure surpasses the value pwcritical, landslide is prone to be induced by rainfall. Consequently rainfall and other condtion changes have important effects inducing landslides.