A freestanding spent fuel interim storage facility is possible to be selected to house the spent fuel for ChinShan nuclear power station in a period of 5 to 50 years. The design specifications of the storage facility should provide adequate safety assurance under the event of a major earthquake. ANSYS/LS-DYNA computer code is employed in this study to simulate the dynamic behavior of the cask under extremely seismic loading, the criterion which is used to judge whether freestanding cask will slide or overturn is thus established. A cylindrical cask is used as an object which subjected to safety shutdown earthquake of support pad of ChinShan nuclear power station. The cylindrical cask will slide but will not overturn is obtained under maximum earthquake loading.