Abstract: Because the lock segment can suddenly snip during the process of high speed landslide in the set-out range, the shearing stress is also suddenly released, which can make the pore water pressure near the sliding zone sharply increase. Therefore it greatly reduces the effective stress and makes the landslide has a very high initial velocity. Taking the Ermanshan landslide as example, this paper employs the field survey, laboratory test and theoretical research methods to study such mechanism. The pore water pressure can be affected by the degree of crushing of slip soil particles, which is analyzed. The condition of producing high pore water pressure when landslide starting is given. On the foundation of theoretical research, it shows that the landslide at Ermanshan site has the condition of producing the high pore water pressure through the exploration. By applying the theory of soil mechanics and the results of mechanics of materials, it can conclude the formula of the pore water pressure in the starting-up of the high speed landslide, and the pore water pressure of Ermanshan landslide are computed by this formula. Furthermore it can deeply investigate the pore water pressure effect in the starting-up of the landslide.