Abstract: Recently the geologic disasters of landslides with debris flows frequently occur and closely relate to earthquakes and rain storms. Thus it is desired to propose a modified calculation method for seismic force. That is, besides the original consideration of the inertia force of earthquake acceleration, another term of the excess seepage force due to the successive vibration of earthquake waves should be added for it can reduce the effective weight of soil mass and make slope easily slide. An example of an infinite landslide with a parallel straight line plane on the weathered mountain slope is calculated. The formulas for shallow slide under rainy saturated and unsaturated conditions are derived respectively. By comparing the calculated results under different slope gradient earthquake accelerations and excess seepage forces, the calculated safety factor has an error of 25% if the excess seepage force is not considered, and it decreases further one half during slope rainy saturation. The sliding soil mass will induce debris flows under rainstorm run-off. Some measures to prevent landslides with successful illustration in engineering practice are proposed.