Abstract: In order to ensure the safety of base isolated buildings during extreme earthquakes, this paper provides a type of stiffness variable isolated protection device which is installed in certain distance from the protected rubber bearings as the second defence measure.And a shaking table test of a five-floor steel-frame model has been completed successfully in Beijing Polytechnic University.On the basis of theory and shaking table test research, the stiffness variable isolated protection device can effectively reduce the horizontal deformation to prescribed limitation and prevent the rubber bearings from failure because the dynamic character of the structure has been changed, and the story shear forces of the super-structure are only slightly increased.