In order to study the effect of the stiffness of flexible bus on the seismic response of the ultra-high voltage (UHV) electrical equipment, the author established a computing formula with considering the bit-shaped function, the maximum sag and the maximum stress of the flexible bus. And the formula was verified by a comparative analysis between the test results and the finite element method (FEM) results. The finite element analysis results show that when the bending stiffness of the flexible bus increases, the sag ratio of the flexible bus reduces, the earthquake coupling function of the flexible bus between UHV equipments and the flexible bus enhances and the tensile effect of the flexible bus to the equipment is reinforced, and thus the seismic response of UHV equipment with flexible bus increases. The effect of the stiffness of the flexible bus on the seismic response of the equipment with high frequency is greater than that of the equipment with low frequency.