Abstract: The great shearing deformation energy is accumulated in locked segment under the action of huge thrust of the upper slope body, and shear strength suddenly drop owing to peak-residual strength drop when sliding surface extend. Once the locked segment is suddenly snipped, the elastic strain energy is converted into the kinetic energy of the sliding mass. In order to study the calculation method of landslide elastic strain energy under seismic load, this paper attempted to obtain the shear strain energy of the locking solid and the peak residual strength reduction strain energy based on elastic mechanics method applying seismic force as physical strength to the micro-body. Consequently, this paper took an attempt to obtain the starting velocity formula of sliding mass on account of Kinetic energy theorem. The new method was verified by an example, and the resultindicates that the speed formula is reasonable.